Metal oxide and field-effect transistor

ABSTRACT

To provide a novel material. In a field-effect transistor including a metal oxide, a channel formation region of the transistor includes a material having at least two different energy band widths. The material includes nano-size particles each with a size of greater than or equal to 0.5 nm and less than or equal to 10 nm. The nano-size particles are dispersed or distributed in a mosaic pattern.

TECHNICAL FIELD

The present invention relates to an object, a method, or a manufacturingmethod. The present invention relates to a process, a machine,manufacture, or a composition of matter. In particular, one embodimentof the present invention relates to a metal oxide or a manufacturingmethod of the metal oxide. In particular, one embodiment of the presentinvention relates to a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a power storage device,a memory device, a method for driving them, or a method formanufacturing them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic appliance may each include a semiconductor device.

BACKGROUND ART

For example, a technique in which a transistor is fabricated using anIn—Ga—Zn-based metal oxide is disclosed (for example, see PatentDocument 1).

Non-Patent Document 1 discusses a structure in which an active layer ofa transistor includes two layers of metal oxides of an In—Zn oxide andan In—Ga—Zn oxide.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-096055

Non-Patent Document

-   [Non-Patent Document 1] John F. Wager, “Oxide TFTs: A Progress    Report”, Information Display 1/16, SID 2016, January/February 2016,    Vol. 32, No. 1, pp. 16-21

DISCLOSURE OF INVENTION

In Non-Patent Document 1, a channel-protective bottom-gate transistorachieves high field-effect mobility (μ=62 cm²V⁻¹s⁻¹). An active layer ofthe transistor is a two-layer stack of an indium zinc oxide and an IGZO,and the thickness of the indium zinc oxide where a channel is formed is10 nm. However, the S value (the subthreshold swing (SS)), which is oneof transistor characteristics, is as large as 0.41 V/decade. Moreover,the threshold voltage (V_(th)), which is also one of transistorcharacteristics, is −2.9 V, which means that the transistor has anormally-on characteristic.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a novel metal oxide. Another object ofone embodiment of the present invention is to give favorable electricalcharacteristics to a semiconductor device. Another object of oneembodiment of the present invention is to provide a highly reliablesemiconductor device. Another object of one embodiment of the presentinvention is to provide a semiconductor device with a novel structure.Another object of one embodiment of the present invention is to providea display device with a novel structure.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

One embodiment of the present invention is a field-effect transistorincluding a metal oxide. A channel formation region of the transistorincludes a material having at least two different energy band widths.The material includes nano-size particles each with a size of greaterthan or equal to 0.5 nm and less than or equal to 10 nm. The nano-sizeparticles are dispersed or distributed in a mosaic pattern.

Another embodiment of the present invention is a field-effect transistorincluding a metal oxide. A channel formation region of the transistorincludes a material having at least two different energy band widths.The material includes nano-size particles with a size of greater than orequal to 0.5 nm and less than or equal to 10 nm. The nano-size particlesare dispersed or distributed in a mosaic pattern. The channel formationregion includes a path through which carriers easily flow from a sourceto a drain.

In the above embodiment, the material having two different energy bandwidths preferably includes a first material having a first energy bandwidth and a second material having a second energy band width. The firstmaterial preferably includes In or Zn. The second material preferablyincludes one or more of In, Zn, Al, Ga, Si, B, Y, Ti, Fe, Ni, Ge, Zr,Mo, La, Ce, Nd, Hf, Ta, W, Mg, V, Be, and Cu. The second energy bandwidth is preferably larger than the first energy band width.

In the above embodiment, the first material preferably includes In andZn, and the second material preferably includes In, Ga, and Zn.

In the above embodiment, the first energy band width is preferably 2.4eV or a value in the vicinity thereof, and the second energy band widthis preferably greater than or equal to 3.0 eV and less than or equal to4.0 eV.

In the above embodiment, the second material preferably includes moreGa, Al, or Si than the first material.

In the above embodiment, the second material preferably includes more Gathan the first material.

Another embodiment of the present invention is a field-effect transistorincluding a metal oxide. A channel formation region of the transistorincludes a material having at least two different energy band widths.The material includes nano-size particles each with a size of greaterthan or equal to 0.5 nm and less than or equal to 10 nm. The nano-sizeparticles are dispersed or distributed in a mosaic pattern. In the casewhere a gate voltage is applied to the transistor in the forwarddirection, the material having at least two different energy band widthshas a first function of allowing each carrier to flow from a source to adrain. In the case where a gate voltage is applied to the transistor inthe reverse direction, the material having at least two different energyband widths has a second function of preventing each carrier fromflowing from the source to the drain.

In the above embodiment, the first function preferably makes thematerial having at least two different energy band widths each narrowerthan an energy band width in the case where the gate voltage is held at0 V, and the second function preferably makes the material having atleast two different energy band widths each wider than an energy bandwidth in the case where the gate voltage is held at 0 V.

In the above embodiment, the nano-size particles preferably include aregion where their surroundings are blurred like a cloud and overlapwith each other.

According to one embodiment of the present invention, a novel metaloxide can be provided. According to one embodiment of the presentinvention, a semiconductor device with favorable electricalcharacteristics can be provided. A highly reliable semiconductor devicecan be provided. A semiconductor device with a novel structure can beprovided. A display device with a novel structure can be provided.

Note that the descriptions of these effects do not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the descriptions of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual view of a composition of a metal oxide.

FIG. 2A is a schematic view of a transistor and FIGS. 2B and 2C areschematic views illustrating distribution of energy levels in thetransistor.

FIGS. 3A to 3C each show a model of a schematic band diagram of atransistor.

FIGS. 4A to 4C each show a model of a schematic band diagram of atransistor.

FIGS. 5A to 5C each show a model of a schematic band diagram of atransistor.

FIGS. 6A to 6C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 7A to 7C are a top view and cross-sectional views of asemiconductor device.

FIGS. 8A and 8B are cross-sectional views illustrating a semiconductordevice.

FIGS. 9A to 9D are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 10A to 10C are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 11A to 11C are cross-sectional views illustrating a method formanufacturing a semiconductor device.

FIGS. 12A to 12C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 13A to 13C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 14A to 14C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 15A to 15C are a top view and cross-sectional views illustrating asemiconductor device.

FIGS. 16A to 16C each illustrate an atomic ratio range of a metal oxideof the present invention.

FIG. 17 illustrates a structure example of a display panel.

FIG. 18 illustrates a structure example of a display panel.

FIGS. 19A and 19B illustrate a model of a metal oxide of one embodimentand its density of states.

FIGS. 20A to 20D illustrate local structures of models of metal oxidesto which impurities are added in one embodiment and their densities ofstates.

FIGS. 21A to 21D illustrate local structures of models of metal oxidesto which impurities are added in one embodiment and their densities ofstates.

FIGS. 22A to 22D illustrate local structures of models of metal oxidesto which impurities are added in one embodiment and their densities ofstates.

FIG. 23 shows measured XRD spectra of samples of Example.

FIGS. 24A to 24F show cross-sectional TEM images and electrondiffraction patterns of samples of Example.

FIGS. 25A to 25L show a plan-view TEM image, a cross-sectional TEMimage, and electron diffraction patterns of a sample of Example.

FIG. 26 shows plan-view TEM images of samples of Example and imagesobtained through analysis thereof.

FIGS. 27A to 27D illustrate a method for deriving a rotation angle of ahexagon.

FIGS. 28A to 28E illustrate a method for forming a Voronoi diagram.

FIG. 29 shows the number of shapes of Voronoi regions of Example andproportions thereof.

FIGS. 30A to 30H show a plan-view TEM image, a cross-sectional TEMimage, and EDX mapping images of a sample of Example.

FIGS. 31A to 31F show EDX mapping images of a sample of Example.

FIGS. 31-2A to 31-9H show plan-view TEM images, cross-sectional TEMimages, and EDX mapping images of samples of Example.

FIG. 32 shows I_(d)-V_(g) characteristics of samples of Example.

FIG. 33 shows I_(d)-V_(g) characteristics of samples after +GBT stresstests of Example.

FIGS. 34A and 34B show I_(d)-V_(g) characteristics and I_(d)-V_(d)characteristics of a transistor.

FIG. 35 shows I_(d)-V_(g) characteristics and linear and saturationmobility curves which are calculated on the basis of GCA.

FIGS. 36A to 36F show cross-sectional TEM images of samples of Exampleand electron diffraction patterns thereof.

FIGS. 37A to 37F show plan-view TEM images of samples of Example andelectron diffraction patterns thereof.

FIGS. 38A to 38C show the numbers of shapes of Voronoi regions ofExample and proportions thereof.

FIGS. 39A to 39H show a plan-view TEM image, a cross-sectional TEMimage, and EDX mapping images of a sample of Example.

FIGS. 40A to 40C show EDX mapping images of a sample of Example.

FIGS. 41A to 41H show a plan-view TEM image, a cross-sectional TEMimage, and EDX mapping images of a sample of Example.

FIG. 42 is a graph showing I_(d)-V_(g) characteristics of samples ofExample.

FIGS. 43A to 43D show a cross-sectional TEM image, EDX mapping images,and a diagram illustrating an atomic ratio of a sample of Example.

FIGS. 44A to 44D show a plan-view TEM image, EDX mapping images, and adiagram illustrating an atomic ratio of a sample of Example.

FIG. 45 shows I_(d)-V_(g) characteristics.

FIG. 46 shows I_(d)-V_(g) characteristics.

FIG. 47 shows calculation results of a density of interface states.

FIGS. 48A and 48B show I_(d)-V_(g) characteristics.

FIG. 49 shows calculation results of a density of defect states.

FIG. 50 shows calculation results of a density of defect states.

FIG. 51 shows I_(d)-V_(g) characteristics of a transistor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with various modes. It willbe readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments.

In the drawings, the size, the layer thickness, or the region isexaggerated for clarity in some cases. Therefore, the size, the layerthickness, or the region is not limited to the illustrated scale. Notethat the drawings are schematic views showing ideal examples, andembodiments of the present invention are not limited to shapes or valuesshown in the drawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

In this specification, terms for describing arrangement, such as “over”,“above”, “under”, and “below”, are used for convenience in describing apositional relation between components with reference to drawings.Furthermore, the positional relation between components is changed asappropriate in accordance with a direction in which each component isdescribed. Thus, there is no limitation on terms used in thisspecification, and description can be made appropriately depending onthe situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. The transistorhas a channel region between a drain (a drain terminal, a drain region,or a drain electrode) and a source (a source terminal, a source region,or a source electrode), and current can flow between the source and thedrain through the channel region. Note that in this specification andthe like, a channel region refers to a region through which currentmainly flows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through an“object having any electric function”. There is no particular limitationon the “object having any electric function” as long as electric signalscan be transmitted and received between components that are connectedthrough the object. Examples of an “object having any electric function”are a switching element such as a transistor, a resistor, an inductor, acapacitor, and an element with a variety of functions as well as anelectrode and a wiring.

In this specification and the like, a “silicon oxynitride film” refersto a film that includes oxygen at a higher proportion than nitrogen, anda “silicon nitride oxide film” refers to a film that includes nitrogenat a higher proportion than oxygen.

In the description of modes of the present invention in thisspecification and the like with reference to the drawings, the samecomponents in different drawings are commonly denoted by the samereference numeral in some cases.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. In addition, the term “substantially parallel” indicatesthat the angle formed between two straight lines is greater than orequal to −30° and less than or equal to 30°. In addition, the term“perpendicular” indicates that the angle formed between two straightlines is greater than or equal to 80° and less than or equal to 100°,and accordingly also includes the case where the angle is greater thanor equal to 85° and less than or equal to 95°. In addition, the term“substantially perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 60° and less than orequal to 120°.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other depending on the case. For example, theterm “conductive layer” can be changed into the term “conductive film”in some cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Note that a “semiconductor” includes characteristics of an “insulator”in some cases when the conductivity is sufficiently low, for example.Furthermore, a “semiconductor” and an “insulator” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “insulator” is not clear. Accordingly, a“semiconductor” in this specification can be called an “insulator” insome cases. Similarly, an “insulator” in this specification can becalled a “semiconductor” in some cases.

Note that in this specification and the like, “In:Ga:Zn=4:2:3 or aneighborhood of In:Ga:Zn=4:2:3” refers to an atomic ratio where, when Inis 4 with respect to the total number of atoms, Ga is greater than orequal to 1 and less than or equal to 3 (1≤Ga≤3) and Zn is greater thanor equal to 2 and less than or equal to 4 (2≤Zn≤4). “In:Ga:Zn=5:1:6 or aneighborhood of In:Ga:Zn=5:1:6” refers to an atomic ratio where, when Inis 5 with respect to the total number of atoms, Ga is greater than 0.1and less than or equal to 2 (0.1<Ga≤2) and Zn is greater than or equalto 5 and less than or equal to 7 (5≤Zn≤7). “In:Ga:Zn=1:1:1 or aneighborhood of In:Ga:Zn=1:1:1” refers to an atomic ratio where, when Inis 1 with respect to the total number of atoms, Ga is greater than 0.1and less than or equal to 2 (0.1<Ga≤2) and Zn is greater than 0.1 andless than or equal to 2 (0.1<Zn≤2).

Embodiment 1

In this embodiment, a metal oxide of one embodiment of the presentinvention is described.

The metal oxide of one embodiment of the present invention preferablycontains at least indium. In particular, indium and zinc are preferablycontained. In addition, an element M (the element M is one or more ofaluminum, gallium, yttrium, copper, vanadium, beryllium, boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like)may be contained.

The metal oxide of one embodiment of the present invention preferablycontains nitrogen. Specifically, the nitrogen concentration in the metaloxide of one embodiment of the present invention measured by secondaryion mass spectrometry (SIMS) may be 1×10¹⁶ atoms/cm³ or higher,preferably 1×10¹⁷ atoms/cm³ or higher and 2×10²² atoms/cm³ or lower.Note that a metal oxide to which nitrogen is added tends to have asmaller band gap and thus have improved conductivity. Thus, in thisspecification and the like, the metal oxide of one embodiment of thepresent invention includes a metal oxide to which nitrogen or the likeis added. Moreover, a metal oxide containing nitrogen may be referred toas metal oxynitride.

Here, the case where the metal oxide contains indium, the element M, andzinc is considered. The terms of the atomic ratio of indium to theelement M and zinc contained in the metal oxide are denoted by [In],[M], and [Zn], respectively.

<Composition of Metal Oxide>

FIG. 1 is a conceptual view of a metal oxide having a cloud-alignedcomplementary (CAC) composition in the present invention. In thisspecification, a metal oxide of one embodiment of the present inventionhaving a semiconductor function is defined as a cloud-alignedcomplementary oxide semiconductor (CAC-OS).

The CAC-OS or the CAC-metal oxide is referred to as a matrix compositeor a metal matrix composite in some cases. Thus, CAC-OS may be called acloud-aligned complementary OS.

For example, in the CAC-OS, as illustrated in FIG. 1, elements includedin the metal oxide are unevenly distributed, and regions 001 mainlyincluding an element and regions 002 mainly including another elementare formed. The regions 001 and 002 are mixed to form a mosaic pattern.In other words, the CAC-OS has a composition in which elements includedin a metal oxide are unevenly distributed. Materials including unevenlydistributed elements each have a size of greater than or equal to 0.5 nmand less than or equal to 10 nm, preferably greater than or equal to 1nm and less than or equal to 2 nm, or a similar size. Note that in thefollowing description of a metal oxide, a state in which one or moremetal elements are unevenly distributed and regions including the metalelement(s) are mixed is referred to as a mosaic pattern or a patch-likepattern. The regions each have a size of greater than or equal to 0.5 nmand less than or equal to 10 nm, preferably greater than or equal to 1nm and less than or equal to 2 nm, or a similar size.

For example, an In-M-Zn oxide having the CAC composition has acomposition in which materials are separated into an indium oxide(InO_(X1), where X1 is a real number greater than 0) or an indium zincoxide (In_(X2)Zn_(Y2)O_(Z2), where X2, Y2, and Z2 are real numbersgreater than 0), an oxide including the element M, and the like, and amosaic pattern is formed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) formingthe mosaic pattern is distributed in the film. Hereinafter, thiscomposition is also referred to as a cloud-like composition.

In other words, the metal oxide of one embodiment of the presentinvention includes at least two oxides or materials selected from an Inoxide, an In-M oxide, an M oxide, an M-Zn oxide, an In—Zn oxide, and anIn-M-Zn oxide.

Typically, the metal oxide of one embodiment of the present inventionincludes at least two or more oxides selected from an In oxide, an In—Znoxide, an In—Al—Zn oxide, an In—Ga—Zn oxide, an In—Y—Zn oxide, anIn—Cu—Zn oxide, an In—V—Zn oxide, an In—Be—Zn oxide, an In—B—Zn oxide,an In—Si—Zn oxide, an In—Ti—Zn oxide, an In—Fe—Zn oxide, an In—Ni—Znoxide, an In—Ge—Zn oxide, an In—Zr—Zn oxide, an In—Mo—Zn oxide, anIn—La—Zn oxide, an In—Ce—Zn oxide, an In—Nd—Zn oxide, an In—Hf—Zn oxide,an In—Ta—Zn oxide, an In—W—Zn oxide, and an In—Mg—Zn oxide. That is, themetal oxide of one embodiment of the present invention can be referredto as a composite metal oxide including a plurality of materials or aplurality of components.

Here, let a concept in FIG. 1 illustrate an In-M-Zn oxide with the CACcomposition. In this case, it can be said that the region 001 is aregion including an oxide including the element M as a main componentand the region 002 is a region including In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component. Surrounding portions of the regionincluding an oxide including the element M as a main component, theregion including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component,and a region including at least Zn are unclear (blurred), so thatboundaries are not clearly observed in some cases.

In other words, an In-M-Zn oxide with the CAC composition is a metaloxide in which a region including an oxide including the element M as amain component and a region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component are mixed. Accordingly, the metal oxide is referredto as a composite metal oxide in some cases. Note that in thisspecification, for example, when the atomic ratio of In to the element Min the region 002 is greater than the atomic ratio of In to the elementMin the region 001, the region 002 has higher In concentration than theregion 001.

Note that in the metal oxide having the CAC composition, a stacked-layerstructure including two or more films with different compositions is notincluded. For example, a two-layer structure of a film including In as amain component and a film including Ga as a main component is notincluded.

Specifically, of the CAC-OS, an In—Ga—Zn oxide with the CAC composition(such an In—Ga—Zn oxide may be particularly referred to as CAC-IGZO) isdescribed. In the In—Ga—Zn oxide including a CAC-OS, materials areseparated into InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) and an oxide includinggallium, for example, and a mosaic pattern is formed. InO_(X1) orIn_(X2)Zn_(Y2)O_(Z2) forming the mosaic pattern is a cloud-like metaloxide.

In other words, an In—Ga—Zn oxide including a CAC-OS is a compositemetal oxide having a composition in which a region including an oxideincluding gallium as a main component and a region includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component are mixed.Surrounding portions of the region including an oxide including galliumas a main component and the region including In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component are unclear (blurred), so that boundariesare not clearly observed in some cases.

For example, in the conceptual view in FIG. 1, the region 001corresponds to the region including an oxide including gallium as a maincomponent and the region 002 corresponds to the region includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component. The regionincluding an oxide including gallium as a main component and the regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component may eachbe referred to as a nanoparticle. The diameter of the nanoparticle isgreater than or equal to 0.5 nm and less than or equal to 10 nm,typically greater than or equal to 1 nm and less than or equal to 2 nm.Surrounding portions of the nanoparticles are unclear (blurred), so thata boundary is not clearly observed in some cases.

The sizes of the region 001 and the region 002 can be measured withenergy dispersive X-ray spectroscopy (EDX) mapping images obtained byEDX. For example, the diameter of the region 001 is greater than orequal to 0.5 nm and less than or equal to 10 nm, or greater than orequal to 1 nm and less than or equal to 2 nm in the EDX mapping image ofa cross-sectional photograph in some cases. The density of an elementwhich is a main component is gradually lowered from the central portionof the region toward the surrounding portion. For example, when thenumber (abundance) of atoms of an element countable in an EDX mappingimage gradually changes from the central portion toward the surroundingportion, the surrounding portion of the region is unclear (blurred) inthe EDX mapping of the cross-sectional photograph. For example, from thecentral portion toward the surrounding portion in the region includingInO_(X1) as a main component, the number of In atoms gradually reducesand the number of Zn atoms gradually increases, so that the regionincluding In_(X2)Zn_(Y2)O_(Z2) as a main component gradually appears.Accordingly, the surrounding portion of a region including InO_(X1) as amain component is unclear (blurred) in the EDX mapping image.

A crystal structure of the In—Ga—Zn oxide with the CAC composition isnot particularly limited. The region 001 and the region 002 may havedifferent crystal structures.

Here, an In—Ga—Zn—O-based metal oxide is referred to as IGZO in somecases, and a compound including In, Ga, Zn, and O is also known as IGZO.A crystalline compound can be given as an example of theIn—Ga—Zn—O-based metal oxide. The crystalline compound has a singlecrystal structure, a polycrystalline structure, or a c-axis alignedcrystalline (CAAC) structure. Note that the CAAC structure is a layeredcrystal structure in which a plurality of IGZO nanocrystals have c-axisalignment and are connected in the a-b plane direction withoutalignment.

Meanwhile, the crystal structure is a secondary element for the In—Ga—Znoxide including a CAC-OS. In this specification, CAC-IGZO can be definedas a metal oxide including In, Ga, Zn, and O in the state where aplurality of regions including Ga as a main component and a plurality ofregions including In as a main component are each dispersed randomlyforming a mosaic pattern.

For example, in the conceptual view in FIG. 1, the region 001corresponds to the region including Ga as a main component and theregion 002 corresponds to the region including In as a main component.The region including Ga as a main component and the region including Inas a main component may each be referred to as a nanoparticle. Thediameter of the nanoparticle is greater than or equal to 0.5 nm and lessthan or equal to 10 nm, typically greater than or equal to 1 nm and lessthan or equal to 2 nm. Surrounding portions of the nanoparticles areunclear (blurred), so that a boundary is not clearly observed in somecases.

The crystallinity of the In—Ga—Zn oxide including a CAC-OS can beanalyzed by electron diffraction. For example, a ring-like region withhigh luminance is observed in an electron diffraction pattern image.Furthermore, a plurality of spots are observed in the ring-shaped regionin some cases.

As described above, the In—Ga—Zn oxide including a CAC-OS has astructure different from that of an IGZO compound in which metalelements are evenly distributed, and has characteristics different fromthose of the IGZO compound. That is, in the In—Ga—Zn oxide including aCAC-OS, regions including an oxide including gallium or the like as amain component and regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) asa main component are separated to form a mosaic pattern.

The conductivity of the region including In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component is higher than that of the region includingan oxide including gallium or the like as a main component. In otherwords, when carriers flow through the regions includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component, the conductivityof an oxide semiconductor is exhibited. Accordingly, when the regionsincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component aredistributed in an oxide semiconductor like a cloud, high field-effectmobility (μ) can be achieved. The region including In_(X2)Zn_(Y2)O_(Z2)or InO_(X1) as a main component can be said to be a semiconductor regionwhose properties are close to those of a conductor.

In contrast, the insulating property of the region including an oxideincluding gallium or the like as a main component is higher than that ofthe region including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a maincomponent. In other words, when the regions including an oxide includinggallium or the like as a main component are distributed in an oxidesemiconductor, leakage current can be suppressed and favorable switchingoperation can be achieved. The region including In_(a)Ga_(b)Zn_(c)O_(d)or the like as a main component can be said to be a semiconductor regionwhose properties are close to those of an insulator.

Accordingly, when the In—Ga—Zn oxide including a CAC-OS is used for asemiconductor element, the insulating property derived from the oxideincluding gallium or the like and the conductivity derived fromIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) complement each other, whereby highon-state current (I_(on)), high field-effect mobility (μ), and lowoff-state current (I_(off)) can be achieved.

A semiconductor element including the In—Ga—Zn oxide including a CAC-OShas high reliability. Thus, the In—Ga—Zn oxide including a CAC-OS issuitably used in a variety of semiconductor devices typified by adisplay.

<Transistor Including Metal Oxide>

Next, the case where the metal oxide is used as a semiconductor in atransistor is described.

With the use of the metal oxide as a semiconductor in a transistor, thetransistor can have high field-effect mobility and high switchingcharacteristics. In addition, the transistor can have high reliability.

FIG. 2A is a schematic view of a transistor including the metal oxide ina channel region. The transistor in FIG. 2A includes a source, a drain,a first gate, a second gate, a first gate insulating portion, a secondgate insulating portion, and a channel portion. The resistance of achannel portion of a transistor can be controlled by application of apotential to a gate. That is, conduction (the on state of thetransistor) or non-conduction (the off state of the transistor) betweenthe source and the drain can be controlled by a potential applied to thefirst gate or the second gate.

The channel portion includes a CAC-OS in which the regions 001 having afirst band gap and the regions 002 having a second band gap aredistributed like a cloud. The first band gap is larger than the secondband gap.

For example, the case where the In—Ga—Zn oxide with the CAC compositionis used as the CAC-OS in the channel portion is described. The In—Ga—Znoxide with the CAC composition has a composition in which materials areseparated into, as the region 001, a region includingIn_(a)Ga_(b)Zn_(c)O_(d) as a main component and having higher Gaconcentration than the region 002, and, as the region 002, a regionincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component andhaving higher In concentration than the region 001, and a mosaic patternis formed. Then, InO_(X1) or In_(X2)Zn_(Y2)O_(Z2) is distributed in thefilm. This composition is also referred to as a cloud-like composition.The region 001 including In_(a)Ga_(b)Zn_(c)O_(d) as a main component hasa band gap larger than that of the region 002 includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component.

A conduction model of the transistor including the CAC-OS in the channelportion illustrated in FIG. 2A is described. FIG. 2B is a schematic viewshowing distribution of energy levels between the source and the drainof the transistor illustrated in FIG. 2A. FIG. 2C is a conduction banddiagram on solid line X-X′ in the transistor illustrated in FIG. 2A.Note that in each conduction band diagram, a solid line indicates theenergy of the conduction band minimum. A dashed line E_(f) indicates theenergy of the quasi-Fermi level of electrons. Here, a negative voltageis applied between the gate and the source as a first gate voltage and adrain voltage (V_(d)>0) is applied between the source and the drain.

When a negative gate voltage is applied to the transistor illustrated inFIG. 2A, an energy of a conduction band minimum CB₀₀₁ derived from theregion 001 and an energy of a conduction band minimum CB₀₀₂ derived fromthe region 002 are formed between the source and the drain asillustrated in FIG. 2B. Since the first band gap is larger than thesecond band gap, the potential barrier of the energy of the conductionband minimum CB₀₀₁ is higher than the potential barrier of the energy ofthe conduction band minimum CB₀₀₂. That is, the maximum value of thepotential barrier in the channel portion is a value derived from theregion 001. Thus, the use of the CAC-OS in the channel portion in atransistor can suppress leakage current and achieve high switchingcharacteristics.

As illustrated in FIG. 2C, the band gap of the region 001 having thefirst band gap is relatively wider than the band gap of the region 002having the second band gap; thus, the Ec edge of the region 001 havingthe first band gap can exist at a relatively higher level than the Ecedge of the region 002 having the second band gap.

For example, it is assumed that a component of the region 001 having thefirst band gap is derived from the In—Ga—Zn oxide (In:Ga:Zn=1:1:1[atomic ratio]), and a component of the region 002 having the secondband gap is derived from an In—Zn oxide (In:Zn=2:3 [atomic ratio]). Inthis case, the first band gap is 3.3 eV or a value in the vicinitythereof, and the second band gap is 2.4 eV or a value in the vicinitythereof. Values obtained by measurement of single films of respectivematerials with an ellipsometer are used as the values of the band gaps.

In the above assumption, the difference between the first band gap andthe second band gap is 0.9 eV. In one embodiment of the presentinvention, the difference between the first band gap and the second bandgap is at least 0.1 eV or more. Note that the position of the valenceband maximum derived from the region 001 having the first band gap isdifferent from the position of the valence band maximum derived from theregion 002 having the second band gap in some cases; thus, thedifference between the first band gap and the second band gap ispreferably 0.3 eV or more, further preferably 0.4 eV or more.

In the above assumption, carriers flow through the CAC-OS owing to anIn—Zn oxide which has the second band gap, i.e., a narrow band gap. Atthis time, the carriers overflow the second band gap into the In—Ga—Znoxide side which has the first band gap, i.e., a wide band gap. In otherwords, carriers are easily generated in an In—Zn oxide which has anarrow band gap, and the carriers move to an In—Ga—Zn oxide which has awide band gap.

In the metal oxide where the channel portion is formed, the regions 001and the regions 002 form a mosaic pattern and are irregularly unevenlydistributed. For this reason, the conduction band diagram on solid lineX-X′ is merely an example.

It is basically acceptable as long as a band in which the region 002 isbetween the regions 001 is formed as shown in FIG. 3A. Alternatively, aband in which the region 001 is between the regions 002 is formed.

In a connection portion of the region 001 having the first band gap andthe region 002 having the second band gap in the CAC-OS, fluctuations inan aggregation state and the composition of the regions occur.Accordingly, as illustrated in FIGS. 3B and 3C, the bands change notdiscontinuously but continuously in some cases. In other words, thefirst band gap and the second band gap work together when carriers flowthrough the CAC-OS.

FIGS. 4A to 4C each show a model of a schematic band diagram of thetransistor in a direction along X-X′ in FIG. 2A, which corresponds tothe schematic view in FIG. 2B. When a voltage is applied to the firstgate electrode, the same voltage is simultaneously applied to the secondgate electrode. FIG. 4A shows a state (on state) in which, as a firstgate voltage V_(g), a positive voltage (V_(g)>0) is applied between eachof the gates and the source. FIG. 4B shows a state in which the firstgate voltage V_(g) is not applied (V_(g)=0). FIG. 4C shows a state (offstate) in which, as the first gate voltage V_(g), a negative voltage(V_(g)<0) is applied between each of the gates and the source. Note thatin a channel portion, a dashed line indicates the energy of theconduction band minimum in the case where no voltage is applied, and asolid line indicates the energy of the conduction band minimum in thecase where a voltage is applied. A dashed-dotted line E_(f) indicatesthe energy of the quasi-Fermi level of electrons.

In a transistor including the CAC-OS in a channel portion, the region001 having the first band gap and the region 002 having the second bandgap electrically interact with each other. In other words, the region001 having the first band gap and the region 002 having the second bandgap function complementarily.

In other words, in the case where a forward voltage is applied as shownin FIG. 4A, the conduction band of the region 001 is lower than theconduction band of the region 002. Accordingly, carriers flow in notonly the conduction band of the region 002 but also the conduction bandof the region 001, so that a high on-state current can be obtained.Meanwhile, in the case where a reverse voltage is applied as shown inFIGS. 4B and 4C, the conduction bands of the regions 001 and 002 becomehigher, which probably causes a significant reduction in the currentflowing between the source and the drain.

FIGS. 5A to 5C show a model of a schematic band diagram of thetransistor in a direction along X-X′ in FIG. 2A, which corresponds tothe schematic view in FIG. 2C. When a voltage is applied to the firstgate electrode, the same voltage is simultaneously applied to the secondgate electrode. FIG. 5A shows a state (on state) in which, as a firstgate voltage V_(g), a positive voltage (V_(g)>0) is applied between eachof the gates and the source. FIG. 5B shows a state in which the firstgate voltage V_(g) is not applied (V_(g)=0). FIG. 5C shows a state (offstate) in which, as the first gate voltage V_(g), a negative voltage(V_(g)<0) is applied between each of the gates and the source. Note thatin a channel portion, a solid line indicates the energy of theconduction band minimum. A dashed-dotted line E_(f) indicates the energyof the quasi-Fermi level of electrons. Here, the energy differencebetween the conduction band minimum of the region 001 and the conductionband minimum of the region 002 is represented as ΔEc. “ΔEc (V_(g)=0)”indicates ΔEc when a voltage is not applied (V_(g)=0), “ΔEc (V_(g)>0)”indicates ΔEc when a voltage at which the transistor is turned on(V_(g)>0) is applied, and “ΔEc (V_(g)<0)” indicates ΔEc when a negativevoltage (V_(g)<0) is applied.

As illustrated in FIG. 5A, when a potential at which the transistor isturned on (V_(g)>0) is applied to the first gate electrode, ΔEc(V_(g)>0)<ΔEc (V_(g)=0) is satisfied. Thus, electrons flow in the region002 having the second band gap with the low Ec edge and serving as amain conduction path. At the same time, electrons also flow in theregion 001 having the first band gap. This enables high current drivecapability in the on state of the transistor, i.e., high on-statecurrent and high field-effect mobility.

In contrast, as illustrated in FIGS. 5B and 5C, when a voltage lowerthan the threshold voltage (V_(g)≤0) is applied to the first gateelectrode, the region 001 having the first band gap serves as adielectric (insulator), so that the conduction path in the region 001 isblocked. The region 002 having the second band gap is in contact withthe region 001 having the first band gap. Consequently, the region 001having the first band gap electrically interacts with the region 002having the second band gap in addition to itself, and thus, even theconduction path in the region 002 having the second band gap is blocked.Accordingly, the whole channel portion is brought into a non-conductivestate, and the transistor is turned off. Therefore, ΔEc (V_(g)=0)<ΔEc(V_(g)<0) is satisfied.

As described above, with the use of the CAC-OS in a transistor, it ispossible to reduce or prevent leakage current between a gate and asource or a drain, which is generated when the transistor operates, forexample, when a potential difference is generated between the gate andthe source or the drain.

A metal oxide with a low carrier density is preferably used in atransistor. A highly purified intrinsic or substantially highly purifiedintrinsic metal oxide has few carrier generation sources and thus canhave a low carrier density. The highly purified intrinsic orsubstantially highly purified intrinsic metal oxide has a low density ofdefect states and accordingly has a low density of trap states in somecases.

Charge trapped by the trap states in the metal oxide takes a long timeto be released and may behave like fixed charge. Thus, a transistorwhose channel region is formed in a metal oxide having a high density oftrap states has unstable electrical characteristics in some cases.

In order to obtain stable electrical characteristics of the transistor,it is effective to reduce the concentration of impurities in the metaloxide. In addition, in order to reduce the concentration of impuritiesin the metal oxide, the concentration of impurities in a film that isadjacent to the metal oxide is preferably reduced. Examples ofimpurities include hydrogen alkali metal, alkaline earth metal, iron,nickel, and silicon.

Here, the influence of impurities in the metal oxide will be described.

When silicon or carbon that is a Group 14 element is contained in themetal oxide, defect states are formed in the metal oxide. Thus, theconcentration of silicon or carbon (measured by secondary ion massspectrometry (SIMS)) is set to be lower than or equal to 2×10¹⁸atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³ in themetal oxide or around an interface with the metal oxide.

When the metal oxide contains an alkali metal or an alkaline earthmetal, defect states are formed and carriers are generated, in somecases. Thus, a transistor including a metal oxide that contains analkali metal or an alkaline earth metal is likely to be normally on.Therefore, it is preferable to reduce the concentration of an alkalimetal or an alkaline earth metal in the metal oxide. Specifically, theconcentration of an alkali metal or an alkaline earth metal in the metaloxide measured by SIMS is set to be lower than or equal to 1×10¹⁸atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³.

Hydrogen included in the metal oxide reacts with oxygen bonded to ametal atom to be water, and thus causes an oxygen vacancy (V_(o)) insome cases. Due to entry of hydrogen into the oxygen vacancy (V_(o)), anelectron serving as a carrier is generated in some cases. Furthermore,in some cases, bonding of part of hydrogen to oxygen bonded to a metalatom causes generation of an electron serving as a carrier. Thus, atransistor including the metal oxide that includes hydrogen is likely tobe normally on. Accordingly, hydrogen in the metal oxide is preferablyreduced as much as possible. Specifically, the hydrogen concentration ofthe metal oxide, which is measured by SIMS, is set to be lower than1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, furtherpreferably lower than 5×10¹⁸ atoms/cm³, still further preferably lowerthan 1×10¹⁸ atoms/cm³.

The oxygen vacancies (V_(o)) in the metal oxide can be reduced byintroduction of oxygen into the metal oxide. That is, the oxygenvacancies (V_(o)) in the metal oxide disappear when the oxygen vacancies(V_(o)) are filled with oxygen. Accordingly, diffusion of oxygen in themetal oxide can reduce the oxygen vacancies (V_(o)) in a transistor andimprove the reliability of the transistor.

As a method for introducing oxygen into the metal oxide, for example, anoxide in which oxygen content is higher than that in the stoichiometriccomposition is provided in contact with the metal oxide. That is, in theoxide, a region including oxygen in excess of that in the stoichiometriccomposition (hereinafter also referred to as an excess oxygen region) ispreferably formed. In particular, in the case of using a metal oxide ina transistor, an oxide including an excess oxygen region is provided ina base film, an interlayer film, or the like in the vicinity of thetransistor, whereby oxygen vacancies in the transistor are reduced, andthe reliability can be improved.

When a metal oxide with sufficiently reduced impurity concentration isused for a channel formation region in a transistor, the transistor canhave stable electrical characteristics.

<Method for Forming Metal Oxide>

An example of a method for forming the metal oxide is described below.

The metal oxide is preferably deposited at a temperature higher than orequal to room temperature and lower than 140° C. Note that roomtemperature includes not only the case where temperature control is notperformed but also the case where temperature control is performed,e.g., the case where a substrate is cooled.

As a sputtering gas, a rare gas (typically argon), oxygen, or a mixedgas of a rare gas and oxygen is used as appropriate. When the mixed gasis used, the proportion of the oxygen gas in the whole deposition gas ishigher than or equal to 0% and lower than or equal to 30%, preferablyhigher than or equal to 5% and lower than or equal to 20%.

When the sputtering gas contains oxygen, oxygen can be added to a filmunder the metal oxide and an excess oxygen region can be provided at thesame time as the deposition of the metal oxide. In addition, increasingthe purity of a sputtering gas is necessary. For example, an oxygen gasor an argon gas used for a sputtering gas is highly purified to have adew point of −40° C. or lower, preferably −80° C. or lower, furtherpreferably −100° C. or lower, still further preferably −120° C. orlower, whereby entry of moisture or the like into the metal oxide can beminimized.

In the case where the metal oxide is deposited by a sputtering method, achamber in a sputtering apparatus is preferably evacuated to a highvacuum (to the degree of approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with anadsorption vacuum pump such as a cryopump in order to remove water orthe like, which serves as an impurity for the metal oxide, as much aspossible. Alternatively, a turbo molecular pump and a cold trap arepreferably combined so as to prevent a backflow of a gas, especially agas containing carbon or hydrogen from an exhaust system to the insideof the chamber.

As a target, an In—Ga—Zn metal oxide target can be used. For example, ametal oxide target having an atomic ratio of [In]:[Ga]:[Zn]=4:2:4.1,[In]:[Ga]:[Zn]=5:1:7, or in the neighborhood thereof is preferably used.

In the sputtering apparatus, the target may be rotated or moved. Forexample, the magnet unit is oscillated vertically and/or horizontallyduring the deposition, whereby the composite metal oxide of the presentinvention can be formed. For example, the target may be rotated oroscillated with a beat (also referred to as rhythm, pulse, frequency,period, cycle, or the like) greater than or equal to 0.1 Hz and lessthan or equal to 1 kHz. Alternatively, the magnet unit may be oscillatedwith a beat of greater than or equal to 0.1 Hz and less than or equal to1 kHz.

The metal oxide of the present invention can be formed, for example, inthe following manner: a mixed gas of oxygen and a rare gas in which theproportion of oxygen is approximately 10% is used; the substratetemperature is 130° C.; and an In—Ga—Zn metal oxide target having anatomic ratio of [In]:[Ga]:[Zn]=4:2:4.1 is oscillated during thedeposition.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments or examples.

Embodiment 2

In this embodiment, semiconductor devices of embodiments of the presentinvention, and manufacturing methods thereof will be described withreference to FIGS. 6A to 6C, FIGS. 7A to 7C, FIGS. 8A and 8B, FIGS. 9Ato 9D, FIGS. 10A to 10C, FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS. 13Ato 13C, FIGS. 14A to 14C, and FIGS. 15A to 15C.

<2-1. Structure Example 1 of Transistor>

FIG. 6A is a top view of a transistor 100 that is a semiconductor deviceof one embodiment of the present invention. FIG. 6B is a cross-sectionalview taken along dashed-dotted line X1-X2 in FIG. 6A. FIG. 6C is across-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 6A.Note that in FIG. 6A, some components of the transistor 100 (e.g., aninsulating film functioning as a gate insulating film) are notillustrated to avoid complexity. The direction of the dashed-dotted lineX1-X2 may be called a channel length direction, and the direction of thedashed-dotted line Y1-Y2 may be called a channel width direction. As inFIG. 6A, some components are not illustrated in some cases in top viewsof transistors described below.

The transistor 100 illustrated in FIGS. 6A to 6C is what is called atop-gate transistor.

The transistor 100 includes an insulating film 104 over a substrate 102,a metal oxide 108 over the insulating film 104, an insulating film 110over the metal oxide 108, a conductive film 112 over the insulating film110, and an insulating film 116 over the insulating film 104, the metaloxide 108, and the conductive film 112.

In a region which overlaps with the conductive film 112, the metal oxide108 is provided over the insulating film 104. For example, the metaloxide 108 preferably contains In, M (M is Al, Ga, Y, or Sn), and Zn.

The metal oxide 108 includes regions 108 n which do not overlap with theconductive film 112 and are in contact with the insulating film 116. Theregions 108 n are n-type regions in the metal oxide 108 described above.The insulating film 116 contains nitrogen or hydrogen. Nitrogen orhydrogen in the insulating film 116 is added to the regions 108 n toincrease the carrier density, thereby making the regions 108 n n-type.

The metal oxide 108 preferably includes a region in which the atomicproportion of In is larger than the atomic proportion of M For example,the atomic ratio of In to M and Zn in the metal oxide 108 is preferablyInM:Zn=4:2:3 or in the neighborhood thereof.

Note that the composition of the metal oxide 108 is not limited to theabove. For example, the atomic ratio of In to M and Zn in the metaloxide 108 is preferably In:M:Zn=5:1:6 or in the neighborhood thereof.The term “neighborhood” includes the following: when In is 5, M isgreater than or equal to 0.5 and less than or equal to 1.5, and Zn isgreater than or equal to 5 and less than or equal to 7.

When the metal oxide 108 has a region in which the atomic proportion ofIn is larger than the atomic proportion of M, the transistor 100 canhave high field-effect mobility. Specifically, the field-effect mobilityof the transistor 100 can exceed 10 cm²/Vs, preferably exceed 30 cm²/Vs.

For example, the use of the transistor with high field-effect mobilityin a gate driver that generates a gate signal allows the display deviceto have a narrow frame. The use of the transistor with high field-effectmobility in a source driver (particularly in a demultiplexer connectedto an output terminal of a shift register included in a source driver)that is included in a display device and supplies a signal from a signalline can reduce the number of wirings connected to the display device.

Even when the metal oxide 108 includes a region in which the atomicproportion of In is higher than the atomic proportion of M, thefield-effect mobility might be low if the metal oxide 108 has highcrystallinity.

Note that the crystallinity of the metal oxide 108 can be determined byanalysis by X-ray diffraction (XRD) or with a transmission electronmicroscope (TEM), for example.

First, oxygen vacancies that might be formed in the metal oxide 108 willbe described.

Oxygen vacancies formed in the metal oxide 108 adversely affect thetransistor characteristics and therefore cause a problem. For example,hydrogen is trapped in oxygen vacancies formed in the metal oxide 108 toserve as a carrier supply source. The carrier supply source generated inthe metal oxide 108 causes a change in the electrical characteristics,typically, shift in the threshold voltage, of the transistor 100including the metal oxide 108. Therefore, it is preferable that theamount of oxygen vacancies in the metal oxide 108 be as small aspossible.

In one embodiment of the present invention, the insulating film in thevicinity of the metal oxide 108 contains excess oxygen. Specifically,one or both of the insulating film 110 which is formed over the metaloxide 108 and the insulating film 104 which is formed below the metaloxide 108 contain excess oxygen. Oxygen or excess oxygen is transferredfrom the insulating film 104 and/or the insulating film 110 to the metaloxide 108, whereby oxygen vacancies in the metal oxide can be reduced.

Impurities such as hydrogen and moisture entering the metal oxide 108adversely affect the transistor characteristics and therefore cause aproblem. Thus, it is preferable that the amount of impurities such ashydrogen and moisture in the metal oxide 108 be as small as possible.

Note that it is preferable to use, as the metal oxide 108, a metal oxidein which the impurity concentration is low and the density of defectstates is low, in which case the transistor can have more excellentelectrical characteristics. Here, the state in which the impurityconcentration is low and the density of defect states is low (the amountof oxygen vacancies is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic metaloxide has few carrier generation sources, and thus can have a lowcarrier density. Thus, a transistor in which a channel region is formedin the metal oxide rarely has a negative threshold voltage (is rarelynormally on). A highly purified intrinsic or substantially highlypurified intrinsic metal oxide has a low density of defect states andaccordingly has a low density of trap states in some cases. Furthermore,the highly purified intrinsic or substantially highly purified intrinsicmetal oxide has an extremely low off-state current; even when an elementhas a channel width of 1×10⁶ μm and a channel length of 10 μm, theoff-state current can be less than or equal to the measurement limit ofa semiconductor parameter analyzer, that is, less than or equal to1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and adrain electrode of from 1 V to 10 V.

As illustrated in FIGS. 6A to 6C, the transistor 100 may further includean insulating film 118 over the insulating film 116, a conductive film120 a electrically connected to the region 108 n through an opening 141a formed in the insulating films 116 and 118; and a conductive film 120b electrically connected to the region 108 n through an opening 141 bformed in the insulating films 116 and 118.

Note that in this specification and the like, the insulating film 104may be referred to as a first insulating film, the insulating film 110may be referred to as a second insulating film, the insulating film 116may be referred to as a third insulating film, and the insulating film118 may be referred to as a fourth insulating film. The conductive films112, 120 a, and 120 b function as a gate electrode, a source electrode,and a drain electrode, respectively.

The insulating film 110 functions as a gate insulating film. Theinsulating film 110 includes an excess oxygen region. Since theinsulating film 110 includes the excess oxygen region, excess oxygen canbe supplied to the metal oxide 108. As a result, oxygen vacancies thatmight be formed in the metal oxide 108 can be filled with excess oxygen,and the semiconductor device can have high reliability.

To supply excess oxygen to the metal oxide 108, excess oxygen may besupplied to the insulating film 104 that is formed below the metal oxide108. In that case, excess oxygen contained in the insulating film 104might also be supplied to the regions 108 n, which is not desirablebecause the resistance of the regions 108 n might be increased. Incontrast, in the structure in which the insulating film 110 formed overthe metal oxide 108 contains excess oxygen, excess oxygen can beselectively supplied only to a region overlapping with the conductivefilm 112.

<2-2. Components of Semiconductor Device>

Next, components of the semiconductor device in this embodiment will bedescribed in detail.

[Substrate]

There is no particular limitation on a material and the like of thesubstrate 102 as long as the material has heat resistance high enough towithstand at least heat treatment to be performed later. For example, aglass substrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like may be used as the substrate 102. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate of silicon or silicon carbide, a compoundsemiconductor substrate of silicon germanium, an SOI substrate, or thelike can be used, or any of these substrates provided with asemiconductor element may be used as the substrate 102. In the casewhere a glass substrate is used as the substrate 102, a glass substratehaving any of the following sizes can be used: the 6th generation (1500mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation(2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10thgeneration (2950 mm×3400 mm). Thus, a large-sized display device can befabricated.

Alternatively, a flexible substrate may be used as the substrate 102,and the transistor 100 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 102 and the transistor 100. The separation layer can be usedwhen part or the whole of a semiconductor device formed over theseparation layer is separated from the substrate 102 and transferredonto another substrate. In such a case, the transistor 100 can betransferred to a substrate having low heat resistance or a flexiblesubstrate as well.

[First Insulating Film]

The insulating film 104 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like as appropriate. Forexample, the insulating film 104 can be formed to have a single-layerstructure or stacked-layer structure including an oxide insulating filmand/or a nitride insulating film. To improve the properties of theinterface with the metal oxide 108, at least a region of the insulatingfilm 104 which is in contact with the metal oxide 108 is preferablyformed using an oxide insulating film. When the insulating film 104 isformed using an oxide insulating film from which oxygen is released byheating, oxygen contained in the insulating film 104 can be moved to themetal oxide 108 by heat treatment.

The thickness of the insulating film 104 can be greater than or equal to50 nm, greater than or equal to 100 nm and less than or equal to 3000nm, or greater than or equal to 200 nm and less than or equal to 1000nm. By increasing the thickness of the insulating film 104, the amountof oxygen released from the insulating film 104 can be increased. Inaddition, interface states at the interface between the insulating film104 and the metal oxide 108 and oxygen vacancies included in the metaloxide 108 can be reduced.

For example, the insulating film 104 can be formed to have asingle-layer structure or stacked-layer structure including siliconoxide, silicon oxynitride, silicon nitride oxide, silicon nitride,aluminum oxide, hafnium oxide, gallium oxide, a Ga—Zn oxide, or thelike. In this embodiment, the insulating film 104 has a stacked-layerstructure including a silicon nitride film and a silicon oxynitridefilm. With the insulating film 104 having such a stack-layer structureincluding a silicon nitride film as a lower layer and a siliconoxynitride film as an upper layer, oxygen can be efficiently introducedinto the metal oxide 108.

[Conductive Film]

The conductive film 112 functioning as a gate electrode and theconductive films 120 a and 120 b functioning as a source electrode and adrain electrode can each be formed using a metal element selected fromchromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc(Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W),manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloyincluding any of these metal elements as its component; an alloyincluding a combination of any of these metal elements; or the like.

Furthermore, the conductive films 112, 120 a, and 120 b can be formedusing an oxide conductor or a metal oxide, such as an oxide includingindium and tin (In—Sn oxide), an oxide including indium and tungsten(In—W oxide), an oxide including indium, tungsten, and zinc (In—W—Znoxide), an oxide including indium and titanium (In—Ti oxide), an oxideincluding indium, titanium, and tin (In—Ti—Sn oxide), an oxide includingindium and zinc (In—Zn oxide), an oxide including indium, tin, andsilicon (In—Sn—Si oxide), or an oxide including indium, gallium, andzinc (In—Ga—Zn oxide).

Here, an oxide conductor is described. In this specification and thelike, an oxide conductor may be referred to as OC. For example, oxygenvacancies are formed in a metal oxide, and then hydrogen is added to theoxygen vacancies, so that a donor level is formed in the vicinity of theconduction band. This increases the conductivity of the metal oxide;accordingly, the metal oxide becomes a conductor. The metal oxide havingbecome a conductor can be referred to as an oxide conductor. Metaloxides generally transmit visible light because of their large energygap. Since an oxide conductor is a metal oxide having a donor level inthe vicinity of the conduction band, the influence of absorption due tothe donor level is small in an oxide conductor, and an oxide conductorhas a visible light transmitting property comparable to that of a metaloxide.

It is particularly preferred to use the oxide conductor described abovefor the conductive film 112, in which case excess oxygen can be added tothe insulating film 110.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedas the conductive films 112, 120 a, and 120 b. The use of a Cu—X alloyfilm results in lower fabrication costs because the film can beprocessed by wet etching.

Among the above-mentioned metal elements, any one or more elementsselected from titanium, tungsten, tantalum, and molybdenum arepreferably included in the conductive films 112, 120 a, and 120 b. Atantalum nitride film is particularly preferable as each of theconductive films 112, 120 a, and 120 b. A tantalum nitride film hasconductivity and a high barrier property against copper or hydrogen.Because a tantalum nitride film releases little hydrogen from itself, itcan be favorably used as the conductive film in contact with the metaloxide 108 or the conductive film in the vicinity of the metal oxide 108.

The conductive films 112, 120 a, and 120 b can be formed by electrolessplating. As a material that can be deposited by electroless plating, forexample, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag,and Pd can be used. It is further favorable to use Cu or Ag because theresistance of the conductive film can be reduced.

[Second Insulating Film]

As the insulating film 110 functioning as a gate insulating film of thetransistor 100, an insulating layer including at least one of thefollowing films formed by a plasma enhanced chemical vapor deposition(PECVD) method, a sputtering method, or the like can be used: a siliconoxide film, a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, a hafnium oxide film, anyttrium oxide film, a zirconium oxide film, a gallium oxide film, atantalum oxide film, a magnesium oxide film, a lanthanum oxide film, acerium oxide film, and a neodymium oxide film. Note that the insulatingfilm 110 may have a two-layer structure or a stacked-layer structureincluding three or more layers.

The insulating film 110 that is in contact with the metal oxide 108functioning as a channel region of the transistor 100 is preferably anoxide insulating film and preferably includes a region including oxygenin excess of the stoichiometric composition (excess oxygen region). Inother words, the insulating film 110 is an insulating film capable ofreleasing oxygen. In order to provide the excess oxygen region in theinsulating film 110, the insulating film 110 is formed in an oxygenatmosphere, or the deposited insulating film 110 is subjected to heattreatment in an oxygen atmosphere, for example.

In the case of using hafnium oxide for the insulating film 110, thefollowing effects are attained. Hafnium oxide has higher dielectricconstant than silicon oxide and silicon oxynitride. Therefore, by usinghafnium oxide, the thickness of the insulating film 110 can be madelarge as compared with the case of using silicon oxide; thus, leakagecurrent due to tunnel current can be low. That is, it is possible toprovide a transistor with a low off-state current. Moreover, hafniumoxide having a crystal structure has a higher dielectric constant thanhafnium oxide having an amorphous structure. Therefore, it is preferableto use hafnium oxide having a crystal structure, in order to obtain atransistor with a low off-state current. Examples of the crystalstructure include a monoclinic crystal structure and a cubic crystalstructure. Note that one embodiment of the present invention is notlimited to the above examples.

It is preferable that the insulating film 110 have few defects andtypically have as few signals observed by electron spin resonance (ESR)spectroscopy as possible. Examples of the signals include a signal dueto an E′ center observed at a g-factor of 2.001. Note that the E′ centeris due to the dangling bond of silicon. As the insulating film 110, asilicon oxide film or a silicon oxynitride film whose spin density of asignal due to the E′ center is lower than or equal to 3×10¹⁷ spins/cm³and preferably lower than or equal to 5×10¹⁶ spins/cm³ may be used.

[Metal Oxide]

As the metal oxide 108, the metal oxide described above can be used.

<Atomic Ratio>

Preferred ranges of the atomic ratio of indium, the element M, and zinccontained in the metal oxide according to the present invention aredescribed with reference to FIGS. 16A to 16C. Note that the proportionof oxygen atoms is not illustrated in FIGS. 16A to 16C. The terms of theatomic ratio of indium, the element M, and zinc contained in the metaloxide are denoted by [In], [M], and [Zn], respectively.

In FIGS. 16A to 16C, dashed lines indicate a line representing theatomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):1 where a is a real numbergreater than or equal to −1 and less than or equal to 1, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):2, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):3, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):4, and a linerepresenting the atomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):5.

Dashed-dotted lines correspond to a line representing the atomic ratioof [In]:[M]:[Zn]=5:1:β where β is a real number greater than or equal to0, a line representing the atomic ratio of [In]:[M]:[Zn]=2:1:β, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=1:1:β, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=1:2:β, a linerepresenting the atomic ratio of [In]:[M]:[Zn]=1:3:β, and a linerepresenting the atomic ratio of [In]:[M]:[Zn]=1:4:β.

A metal oxide having the atomic ratio of [In]:[M]:[Zn]=0:2:1 or in theneighborhood thereof in FIGS. 16A to 16C tends to have a spinel crystalstructure.

A plurality of phases (e.g., two phases or three phases) exist in themetal oxide in some cases. For example, with an atomic ratio[In]:[M]:[Zn] that is close to 0:2:1, two phases of a spinel crystalstructure and a layered crystal structure are likely to exist. Inaddition, with an atomic ratio [In]:[M]:[Zn] that is close to 1:0:0, twophases of a bixbyite crystal structure and a layered crystal structureare likely to exist. In the case where a plurality of phases exist inthe metal oxide, a grain boundary might be formed between differentcrystal structures.

A region A in FIG. 16A shows an example of the preferred ranges of theatomic ratio of indium to the element M and zinc contained in a metaloxide.

In addition, the metal oxide containing indium in a higher proportioncan have high carrier mobility (electron mobility). Thus, a metal oxidehaving a high content of indium has higher carrier mobility than a metaloxide having a low content of indium.

In contrast, when the indium content and the zinc content in a metaloxide become lower, carrier mobility becomes lower. Thus, with an atomicratio of [In]:[M]:[Zn]=0:1:0 and the neighborhood thereof (e.g., aregion C in FIG. 16C), insulation performance becomes better.

Accordingly, the metal oxide of one embodiment of the present inventionpreferably has an atomic ratio represented by the region A in FIG. 16A.With the atomic ratio, high carrier mobility is obtained.

A metal oxide having an atomic ratio in the region A, particularly in aregion B in FIG. 16B, has high carrier mobility and high reliability andis excellent.

Note that the region B includes an atomic ratio of [In]:[M]:[Zn]=4:2:3to 4:2:4.1 and the vicinity thereof. The vicinity includes an atomicratio of [In]:[M]:[Zn]=5:3:4. Note that the region B includes an atomicratio of [In]:[M]:[Zn]=5:1:6 and the vicinity thereof and an atomicratio of [In]:[M]:[Zn]=5:1:7 and the vicinity thereof.

Note that the property of a metal oxide is not uniquely determined by anatomic ratio. Even with the same atomic ratio, the property of a metaloxide might be different depending on a formation condition. Forexample, in the case where the metal oxide is deposited with asputtering apparatus, a film having an atomic ratio deviated from theatomic ratio of a target is formed. In particular, [Zn] in the filmmight be smaller than [Zn] in the target depending on the substratetemperature in deposition. Thus, the illustrated regions each representan atomic ratio with which a metal oxide tends to have specificcharacteristics, and boundaries of the regions A to C are not clear.

In the case where the metal oxide 108 is formed of In-M-Zn oxide, it ispreferable to use a target including polycrystalline In-M-Zn oxide asthe sputtering target. Note that the atomic ratio of metal elements inthe formed metal oxide 108 varies from the above atomic ratios of metalelements of the sputtering targets in a range of ±40%. For example, whena sputtering target used for the metal oxide 108 has an atomic ratio ofIn:Ga:Zn=4:2:4.1, the atomic ratio of the metal oxide 108 may be 4:2:3or in the neighborhood thereof. When a sputtering target used for themetal oxide 108 has an atomic ratio of In:Ga:Zn=5:1:7, the atomic ratioof the metal oxide 108 may be 5:1:6 or in the neighborhood thereof.

The energy gap of the metal oxide 108 is 2 eV or more, preferably 2.5 eVor more. With the use of a metal oxide having such a wide energy gap,the off-state current of the transistor 100 can be reduced.

Furthermore, the metal oxide 108 may have a non-single-crystalstructure. Examples of the non-single-crystal structure include ac-axis-aligned crystalline oxide semiconductor (CAAC-OS) which will bedescribed later, a polycrystalline structure, a microcrystallinestructure, and an amorphous structure. Among the non-single-crystalstructures, an amorphous structure has the highest density of defectstates.

[Third Insulating Film]

The insulating film 116 contains nitrogen or hydrogen. A nitrideinsulating film can be used as the insulating film 116, for example. Thenitride insulating film can be formed using silicon nitride, siliconnitride oxide, silicon oxynitride, or the like. The hydrogenconcentration in the insulating film 116 is preferably higher than orequal to 1×10²² atoms/cm³. The insulating film 116 is in contact withthe region 108 n of the metal oxide 108. Thus, the concentration of animpurity (e.g., hydrogen) in the region 108 n in contact with theinsulating film 116 is increased, leading to an increase in the carrierdensity of the region 108 n.

[Fourth Insulating Film]

As the insulating film 118, an oxide insulating film can be used.Alternatively, a layered film of an oxide insulating film and a nitrideinsulating film can be used as the insulating film 118. The insulatingfilm 118 can be formed using, for example, silicon oxide, siliconoxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide,gallium oxide, or Ga—Zn oxide.

Furthermore, the insulating film 118 preferably functions as a barrierfilm against hydrogen, water, and the like from the outside.

The thickness of the insulating film 118 can be greater than or equal to30 nm and less than or equal to 500 nm, or greater than or equal to 100nm and less than or equal to 400 nm.

<2-3. Structure Example 2 of Transistor>

Next, a structure of a transistor different from that in FIGS. 6A to 6Cwill be described with reference to FIGS. 7A to 7C.

FIG. 7A is a top view of the transistor 150. FIG. 7B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 7A.FIG. 7C is a cross-sectional view taken along dashed-dotted line Y1-Y2in FIG. 7A.

The transistor 150 illustrated in FIGS. 7A to 7C includes the conductivefilm 106 over the substrate 102; the insulating film 104 over theconductive film 106; the metal oxide 108 over the insulating film 104;the insulating film 110 over the metal oxide 108; the conductive film112 over the insulating film 110; and the insulating film 116 over theinsulating film 104, the metal oxide 108, and the conductive film 112.

Note that the metal oxide 108 has a structure similar that in thetransistor 100 shown in FIGS. 6A to 6C. The transistor 150 shown inFIGS. 7A to 7C includes the conductive film 106 and an opening 143 inaddition to the components of the transistor 100 described above.

The opening 143 is provided in the insulating films 104 and 110. Theconductive film 106 is electrically connected to the conductive film 112through the opening 143. Thus, the same potential is applied to theconductive film 106 and the conductive film 112. Note that differentpotentials may be applied to the conductive film 106 and the conductivefilm 112 without providing the opening 143. Alternatively, theconductive film 106 may be used as a light-blocking film withoutproviding the opening 143. When the conductive film 106 is formed usinga light-blocking material, for example, light from the bottom thatirradiates a second region can be reduced.

In the case of the structure of the transistor 150, the conductive film106 functions as a first gate electrode (also referred to as abottom-gate electrode), the conductive film 112 functions as a secondgate electrode (also referred to as a top-gate electrode), theinsulating film 104 functions as a first gate insulating film, and theinsulating film 110 functions as a second gate insulating film.

The conductive film 106 can be formed using a material similar to theabove-described materials of the conductive films 112, 120 a, and 120 b.It is particularly suitable to use a material containing copper as theconductive film 106 because the resistance can be reduced. It isfavorable that, for example, each of the conductive films 106, 120 a,and 120 b has a stacked-layer structure in which a copper film is over atitanium nitride film, a tantalum nitride film, or a tungsten film. Inthat case, by using the transistor 150 as a pixel transistor and/or adriving transistor of a display device, parasitic capacitance generatedbetween the conductive films 106 and 120 a and between the conductivefilms 106 and 120 b can be reduced. Thus, the conductive films 106, 120a, and 120 b can be used not only as the first gate electrode, thesource electrode, and the drain electrode of the transistor 150, butalso as power supply wirings, signal supply wirings, connection wirings,or the like of the display device.

In this manner, unlike the transistor 100 described above, thetransistor 150 in FIGS. 7A to 7C has a structure in which a conductivefilm functioning as a gate electrode is provided over and under themetal oxide 108. As in the transistor 150, a semiconductor device of oneembodiment of the present invention may have a plurality of gateelectrodes.

As illustrated in FIGS. 7B and 7C, the metal oxide 108 faces theconductive film 106 functioning as a first gate electrode and theconductive film 112 functioning as a second gate electrode and ispositioned between the two conductive films functioning as the gateelectrodes.

Furthermore, the length of the conductive film 112 in the channel widthdirection is larger than the length of the metal oxide 108 in thechannel width direction. In the channel width direction, the whole metaloxide 108 is covered with the conductive film 112 with the insulatingfilm 110 placed therebetween. Since the conductive film 112 is connectedto the conductive film 106 through the opening 143 provided in theinsulating films 104 and 110, a side surface of the metal oxide 108 inthe channel width direction faces the conductive film 112 with theinsulating film 110 placed therebetween.

In other words, the conductive film 106 and the conductive film 112 areconnected through the opening 143 provided in the insulating films 104and 110, and each include a region positioned outside an edge portion ofthe metal oxide 108.

Such a structure enables the metal oxide 108 included in the transistor150 to be electrically surrounded by electric fields of the conductivefilm 106 functioning as a first gate electrode and the conductive film112 functioning as a second gate electrode. A device structure of atransistor, like that of the transistor 150, in which electric fields ofthe first gate electrode and the second gate electrode electricallysurround the metal oxide 108 in which a channel region is formed can bereferred to as a surrounded channel (S-channel) structure.

Since the transistor 150 has the S-channel structure, an electric fieldfor inducing a channel can be effectively applied to the metal oxide 108by the conductive film 106 or the conductive film 112; thus, the currentdrive capability of the transistor 150 can be improved and high on-statecurrent characteristics can be obtained. As a result of the highon-state current, it is possible to reduce the size of the transistor150. Furthermore, since the transistor 150 has a structure in which themetal oxide 108 is surrounded by the conductive film 106 and theconductive film 112, the mechanical strength of the transistor 150 canbe increased.

When seen in the channel width direction of the transistor 150, anopening different from the opening 143 may be formed on the side of themetal oxide 108 on which the opening 143 is not formed.

When a transistor has a pair of gate electrodes between which asemiconductor film is positioned as in the transistor 150, one of thegate electrodes may be supplied with a signal A, and the other gateelectrode may be supplied with a fixed potential V_(b). Alternatively,one of the gate electrodes may be supplied with the signal A, and theother gate electrode may be supplied with a signal B. Alternatively, oneof the gate electrodes may be supplied with a fixed potential V_(a), andthe other gate electrode may be supplied with the fixed potential V_(b).

The signal A is, for example, a signal for controlling the on/off state.The signal A may be a digital signal with two kinds of potentials, apotential V1 and a potential V2 (V1>V2). For example, the potential V1can be a high power supply potential, and the potential V2 can be a lowpower supply potential. The signal A may be an analog signal.

The fixed potential V_(b) is, for example, a potential for controlling athreshold voltage V_(thA) of the transistor. The fixed potential V_(b)may be the potential V1 or the potential V2. In that case, a potentialgenerator circuit for generating the fixed potential V_(b) is notnecessary, which is preferable. The fixed potential V_(b) may bedifferent from the potential V1 or the potential V2. When the fixedpotential V_(b) is low, the threshold voltage V_(thA) can be high insome cases. As a result, the drain current flowing when the gate-sourcevoltage V_(gs) is 0 V can be reduced, and leakage current in a circuitincluding the transistor can be reduced in some cases. The fixedpotential V_(b) may be, for example, lower than the low power supplypotential. Meanwhile, a high fixed potential V_(b) can lower thethreshold voltage V_(thA) in some cases. As a result, the drain currentflowing when the gat-source voltage V_(gs) is a high power supplypotential and the operating speed of the circuit including thetransistor can be increased in some cases. The fixed potential V_(b) maybe, for example, higher than the low power supply potential.

The signal B is, for example, a signal for controlling the on/off state.The signal B may be a digital signal with two kinds of potentials, apotential V3 and a potential V4 (V3>V4). For example, the potential V3can be a high power supply potential, and the potential V4 can be a lowpower supply potential. The signal B may be an analog signal.

When both the signal A and the signal B are digital signals, the signalB may have the same digital value as the signal A. In this case, it maybe possible to increase the on-state current of the transistor and theoperating speed of the circuit including the transistor. Here, thepotential V1 and the potential V2 of the signal A may be different fromthe potential V3 and the potential V4 of the signal B. For example, if agate insulating film for the gate to which the signal B is input isthicker than a gate insulating film for the gate to which the signal Ais input, the potential amplitude of the signal B (V3−V4) may be largerthan the potential amplitude of the signal A (V1−V2). In this manner,the influence of the signal A and that of the signal B on the on/offstate of the transistor can be substantially the same in some cases.

When both the signal A and the signal B are digital signals, the signalB may have a digital value different from that of the signal A. In thiscase, the signal A and the signal B can separately control thetransistor, and thus, higher performance can be achieved. The transistorwhich is, for example, an n-channel transistor can function by itself asa NAND circuit, a NOR circuit, or the like in the following case: thetransistor is turned on only when the signal A has the potential V1 andthe signal B has the potential V3, or the transistor is turned off onlywhen the signal A has the potential V2 and the signal B has thepotential V4. The signal B may be a signal for controlling the thresholdvoltage V_(thA). For example, the potential of the signal B in a periodin which the circuit including the transistor operates may be differentfrom the potential of the signal B in a period in which the circuit doesnot operate. The potential of the signal B may vary depending on theoperation mode of the circuit. In this case, the potential of the signalB is not changed as frequently as the potential of the signal A in somecases.

When both the signal A and the signal B are analog signals, the signal Bmay be an analog signal having the same potential as the signal A, ananalog signal whose potential is a constant times the potential of thesignal A, an analog signal whose potential is higher or lower than thepotential of the signal A by a constant, or the like. In this case, itmay be possible to increase the on-state current of the transistor andthe operating speed of the circuit including the transistor. The signalB may be an analog signal different from the signal A. In this case, thesignal A and the signal B can separately control the transistor, andthus, higher performance can be achieved.

The signal A may be a digital signal, and the signal B may be an analogsignal. Alternatively, the signal A may be an analog signal, and thesignal B may be a digital signal.

When both of the gate electrodes of the transistor are supplied with thefixed potentials, the transistor can function as an element equivalentto a resistor in some cases. For example, in the case where thetransistor is an n-channel transistor, the effective resistance of thetransistor can be sometimes low (high) when the fixed potential V_(a) orthe fixed potential V_(b) is high (low). When both the fixed potentialV_(a) and the fixed potential V_(b) are high (low), the effectiveresistance can be lower (higher) than that of a transistor with only onegate in some cases.

The other components of the transistor 150 are similar to those of thetransistor 100 described above and have similar effects.

An insulating film may further be formed over the transistor 150. Thetransistor 150 illustrated in FIGS. 7A to 7C includes an insulating film122 over the conductive films 120 a and 120 b and the insulating film118.

The insulating film 122 has a function of covering unevenness and thelike caused by the transistor or the like. The insulating film 122 hasan insulating property and is formed using an inorganic material or anorganic material. Examples of the inorganic material include a siliconoxide film, a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, and an aluminum nitridefilm. Examples of the organic material include photosensitive resinmaterials such as an acrylic resin and a polyimide resin.

<2-4. Structure Example 3 of Transistor>

Next, a structure of a transistor different from that of the transistor150 in FIGS. 7A to 7C will be described with reference to FIGS. 8A and8B.

FIGS. 8A and 8B are cross-sectional views of a transistor 160. The topview of the transistor 160 is not illustrated because it is similar tothat of the transistor 150 in FIG. 7A.

The transistor 160 illustrated in FIGS. 8A and 8B is different from thetransistor 150 in the stacked-layer structure of the conductive film112, the shape of the conductive film 112, and the shape of theinsulating film 110.

The conductive film 112 in the transistor 160 includes a conductive film112_1 over the insulating film 110 and a conductive film 112_2 over theconductive film 112_1. For example, an oxide conductive film is used asthe conductive film 112_1, so that excess oxygen can be added to theinsulating film 110. The oxide conductive film can be formed by asputtering method in an atmosphere containing an oxygen gas. As theoxide conductive film, an oxide including indium and tin, an oxideincluding tungsten and indium, an oxide including tungsten, indium, andzinc, an oxide including titanium and indium, an oxide includingtitanium, indium, and tin, an oxide including indium and zinc, an oxideincluding silicon, indium, and tin, an oxide including indium, gallium,and zinc, or the like can be used, for example.

As illustrated in FIG. 8B, the conductive film 112_2 is connected to theconductive film 106 through the opening 143. By forming the opening 143after a conductive film to be the conductive film 112_1 is formed, theshape illustrated in FIG. 8B can be obtained. In the case where an oxideconductive film is used as the conductive film 112_1, the structure inwhich the conductive film 112_2 is connected to the conductive film 106can decrease the contact resistance between the conductive film 112 andthe conductive film 106.

The conductive film 112 and the insulating film 110 in the transistor160 have a tapered shape. More specifically, the lower edge portion ofthe conductive film 112 is positioned outside the upper edge portion ofthe conductive film 112. The lower edge portion of the insulating film110 is positioned outside the upper edge portion of the insulating film110. In addition, the lower edge portion of the conductive film 112 isformed in substantially the same position as that of the upper edgeportion of the insulating film 110.

As compared with the transistor 160 in which the conductive film 112 andthe insulating film 110 have a rectangular shape, the transistor 160 inwhich the conductive film 112 and the insulating film 110 have a taperedshape is favorable because of better coverage with the insulating film116.

The other components of the transistor 160 are similar to those of thetransistor 150 described above and have similar effects.

<2-5. Method for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 150 illustrated in FIGS.7A to 7C will be described with reference to FIGS. 9A to 9D, FIGS. 10Ato 10C, and FIGS. 11A to 11C. Note that FIGS. 9A to 9D, FIGS. 10A to10C, and FIGS. 11A to 11C are cross-sectional views in the channellength direction and the channel width direction illustrating the methodfor manufacturing the transistor 150.

First, the conductive film 106 is formed over the substrate 102. Next,the insulating film 104 is formed over the substrate 102 and theconductive film 106, and a metal oxide film is formed over theinsulating film 104. Then, the metal oxide film is processed into anisland shape, whereby a metal oxide 108 a is formed (see FIG. 9A).

The conductive film 106 can be formed using a material selected from theabove-mentioned materials. In this embodiment, for the conductive film106, a stack including a 50-nm-thick tungsten film and a 400-nm-thickcopper film is formed with a sputtering apparatus.

To process a conductive film to be the conductive film 106, a wetetching method and/or a dry etching method can be used. In thisembodiment, in the processing of the conductive film into the conductivefilm 106, the copper film is etched by a wet etching method, and thenthe tungsten film is etched by a dry etching method.

The insulating film 104 can be formed by a sputtering method, a CVDmethod, an evaporation method, a pulsed laser deposition (PLD) method, aprinting method, a coating method, or the like as appropriate. In thisembodiment, as the insulating film 104, a 400-nm-thick silicon nitridefilm and a 50-nm-thick silicon oxynitride film are formed with a PECVDapparatus.

After the insulating film 104 is formed, oxygen may be added to theinsulating film 104. As oxygen added to the insulating film 104, anoxygen radical, an oxygen atom, an oxygen atomic ion, an oxygenmolecular ion, or the like may be used. Oxygen can be added by an iondoping method, an ion implantation method, a plasma treatment method, orthe like. Alternatively, a film that suppresses oxygen release may beformed over the insulating film 104, and then oxygen may be added to theinsulating film 104 through the film.

The film that suppresses oxygen release can be formed using a conductivefilm or a semiconductor film containing one or more of indium, zinc,gallium, tin, aluminum, chromium, tantalum, titanium, molybdenum,nickel, iron, cobalt, and tungsten.

In the case where oxygen is added by plasma treatment in which oxygen isexcited by a microwave to generate high-density oxygen plasma, theamount of oxygen added to the insulating film 104 can be increased.

In forming the metal oxide 108 a, an inert gas (such as a helium gas, anargon gas, or a xenon gas) may be mixed into the oxygen gas. Note thatthe proportion of the oxygen gas in the whole deposition gas(hereinafter also referred to as an oxygen flow rate ratio) in formingthe metal oxide 108 a is higher than or equal to 0% and lower than orequal to 30%, preferably higher than or equal to 5% and lower than orequal to 20%.

The metal oxide 108 a is formed at a substrate temperature higher thanor equal to room temperature and lower than or equal to 180° C.,preferably higher than or equal to room temperature and lower than orequal to 140° C. The substrate temperature when the metal oxide 108 a isformed is preferably, for example, higher than or equal to roomtemperature and lower than 140° C. because the productivity isincreased.

The thickness of the metal oxide 108 a is greater than or equal to 3 nmand less than or equal to 200 nm, preferably greater than or equal to 3nm and less than or equal to 100 nm, further preferably greater than orequal to 3 nm and less than or equal to 60 nm.

In the case where a large-sized glass substrate (e.g., the 6thgeneration to the 10th generation) is used as the substrate 102 and themetal oxide 108 a is formed at a substrate temperature higher than orequal to 200° C. and lower than or equal to 300° C., the substrate 102might be changed in shape (distorted or warped). Therefore, in the casewhere a large-sized glass substrate is used, the change in the shape ofthe glass substrate can be suppressed by forming the metal oxide 108 aat a substrate temperature higher than or equal to room temperature andlower than 200° C.

In addition, increasing the purity of the sputtering gas is necessary.For example, when a gas which is highly purified to have a dew point of−40° C. or lower, preferably −80° C. or lower, further preferably −100°C. or lower, still further preferably −120° C. or lower, is used as thesputtering gas, i.e., the oxygen gas or the argon gas, entry of moistureor the like into the metal oxide can be minimized.

In the case where the metal oxide is deposited by a sputtering method, achamber in a sputtering apparatus is preferably evacuated to be a highvacuum state (to the degree of about 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) with anadsorption vacuum evacuation pump such as a cryopump in order to removewater or the like, which serves as an impurity for the metal oxide, asmuch as possible. In particular, the partial pressure of gas moleculescorresponding to H₂O (gas molecules corresponding to m/z=18) in thechamber in the standby mode of the sputtering apparatus is preferablylower than or equal to 1×10⁻⁴ Pa, further preferably lower than or equalto 5×10⁻⁵ Pa.

In this embodiment, the metal oxide 108 a is formed in the followingconditions.

The metal oxide 108 a is formed by a sputtering method using an In—Ga—Znmetal oxide target. The substrate temperature and the oxygen flow rateat the time of formation of the metal oxide 108 a can be set asappropriate. The pressure in a chamber is 0.6 Pa, and an AC power of2500 W is supplied to the metal oxide target provided in the sputteringapparatus.

To process the metal oxide into the metal oxide 108 a, a wet etchingmethod and/or a dry etching method can be used.

After the metal oxide 108 a is formed, the metal oxide 108 a may bedehydrated or dehydrogenated by heat treatment. The temperature of theheat treatment is typically higher than or equal to 150° C. and lowerthan the strain point of the substrate, higher than or equal to 250° C.and lower than or equal to 450° C., or higher than or equal to 300° C.and lower than or equal to 450° C.

The heat treatment can be performed in an inert gas atmospherecontaining nitrogen or a rare gas such as helium, neon, argon, xenon, orkrypton. Alternatively, the heat treatment may be performed in an inertgas atmosphere first, and then in an oxygen atmosphere. It is preferablethat the above inert gas atmosphere and the above oxygen atmosphere notcontain hydrogen, water, or the like. The treatment time may be longerthan or equal to 3 minutes and shorter than or equal to 24 hours.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment. With the use of an RTA apparatus, the heat treatment canbe performed at a temperature higher than or equal to the strain pointof the substrate if the heating time is short. Therefore, the heattreatment time can be shortened.

By depositing the metal oxide while it is heated or by performing heattreatment after the deposition of the metal oxide, the hydrogenconcentration in the metal oxide, which is measured by SIMS, can be5×10¹⁹ atoms/cm³ or lower, 1×10¹⁹ atoms/cm³ or lower, 5×10¹⁸ atoms/cm³or lower, 1×10¹⁸ atoms/cm³ or lower, 5×10¹⁷ atoms/cm³ or lower, or1×10¹⁶ atoms/cm³ or lower.

Next, an insulating film 110_0 is formed over the insulating film 104and the metal oxide 108 a (see FIG. 9B).

For the insulating film 110_0, a silicon oxide film or a siliconoxynitride film can be formed with a plasma-enhanced chemical vapordeposition apparatus (also referred to as a PECVD apparatus or simply aplasma CVD apparatus). In this case, a deposition gas containing siliconand an oxidizing gas are preferably used as a source gas. Typicalexamples of the deposition gas containing silicon include silane,disilane, trisilane, and silane fluoride. Examples of the oxidizing gasinclude oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide.

A silicon oxynitride film having few defects can be formed as theinsulating film 110_0 with the PECVD apparatus under the conditions thatthe flow rate of the oxidizing gas is more than 20 times and less than100 times, or more than or equal to 40 times and less than or equal to80 times the flow rate of the deposition gas and that the pressure in atreatment chamber is lower than 100 Pa or lower than or equal to 50 Pa.

As the insulating film 110_0, a dense silicon oxide film or a densesilicon oxynitride film can be formed under the following conditions:the substrate placed in a vacuum-evacuated treatment chamber of thePECVD apparatus is held at a temperature higher than or equal to 280° C.and lower than or equal to 400° C.; the pressure in the treatmentchamber into which a source gas is introduced is set to be higher thanor equal to 20 Pa and lower than or equal to 250 Pa, preferably higherthan or equal to 100 Pa and lower than or equal to 250 Pa; and ahigh-frequency power is supplied to an electrode provided in thetreatment chamber.

The insulating film 110_0 may be formed by a PECVD method using amicrowave. A microwave refers to a wave in the frequency range of 300MHz to 300 GHz. In the case of using a microwave, electron temperatureand electron energy are low. Furthermore, in supplied power, theproportion of power used for acceleration of electrons is low, andtherefore, much more power can be used for dissociation and ionizationof molecules. Thus, plasma with a high density (high-density plasma) canbe excited. This method causes little plasma damage to the depositionsurface or a deposit, so that the insulating film 110_0 having fewdefects can be formed.

Alternatively, the insulating film 110_0 can also be formed by a CVDmethod using an organosilane gas. As the organosilane gas, any of thefollowing silicon-containing compound can be used: tetraethylorthosilicate (TEOS) (chemical formula: Si(OC₂H₅)₄); tetramethylsilane(TMS) (chemical formula: Si(CH₃)₄); tetramethylcyclotetrasiloxane(TMCTS); octamethylcyclotetrasiloxane (OMCTS); hexamethyldisilazane(HMDS); triethoxysilane (SiH(OC₂H₅)₃); trisdimethylaminosilane(SiH(N(CH₃)₂)₃); or the like. The insulating film 110_0 having highcoverage can be formed by a CVD method using an organosilane gas.

In this embodiment, as the insulating film 110_0, a 100-nm-thick siliconoxynitride film is formed with the PECVD apparatus.

Subsequently, a mask is formed by lithography in a desired position overthe insulating film 110_0, and then the insulating film 110_0 and theinsulating film 104 are partly etched, so that the opening 143 reachingthe conductive film 106 is formed (see FIG. 9C).

To form the opening 143, a wet etching method and/or a dry etchingmethod can be used. In this embodiment, the opening 143 is formed by adry etching method.

Next, a conductive film 112_0 is formed over the conductive film 106 andthe insulating film 110_0 so as to cover the opening 143. In the casewhere a metal oxide film is used as the conductive film 112_0, forexample, oxygen might be added to the insulating film 110_0 during theformation of the conductive film 112_0 (see FIG. 9D).

In FIG. 9D, oxygen added to the insulating film 110_0 is schematicallyshown by arrows. Furthermore, the conductive film 112_0 formed to coverthe opening 143 is electrically connected to the conductive film 106.

In the case where a metal oxide film is used as the conductive film112_0, the conductive film 112_0 is preferably formed by a sputteringmethod in an atmosphere containing an oxygen gas. Formation of theconductive film 112_0 in an atmosphere containing an oxygen gas allowssuitable addition of oxygen to the insulating film 110_0. Note that amethod for forming the conductive film 112_0 is not limited to asputtering method, and another method such as an ALD method may be used.

In this embodiment, a 100-nm-thick IGZO film containing an In—Ga—Znoxide (In:Ga:Zn=4:2:4.1 (atomic ratio)) is formed as the conductive film112_0 by a sputtering method. Oxygen addition treatment may be performedon the insulating film 110_0 before or after the formation of theconductive film 112_0. The oxygen addition treatment can be performed ina manner similar to that of the oxygen addition treatment that can beperformed after the formation of the insulating film 104.

Subsequently, a mask 140 is formed by a lithography process in a desiredposition over the conductive film 112_0 (see FIG. 10A).

Next, etching is performed from above the mask 140 to process theconductive film 112_0 and the insulating film 110_0. After theprocessing of the conductive film 112_0 and the insulating film 110_0,the mask 140 is removed. As a result of the processing of the conductivefilm 112_0 and the insulating film 110_0, the island-shaped conductivefilm 112 and the island-shaped insulating film 110 are formed (see FIG.10B).

In this embodiment, the conductive film 112_0 and the insulating film110_0 are processed by a dry etching method.

In the processing of the conductive film 112_0 and the insulating film110_0, the thickness of the metal oxide 108 a in a region notoverlapping with the conductive film 112 is decreased in some cases. Inother cases, in the processing of the conductive film 112_0 and theinsulating film 110_0, the thickness of the insulating film 104 in aregion not overlapping with the metal oxide 108 a is decreased. In theprocessing of the conductive film 112_0 and the insulating film 110_0,an etchant or an etching gas (e.g., chlorine) might be added to themetal oxide 108 a or the constituent element of the conductive film112_0 or the insulating film 110_0 might be added to the metal oxide108.

Next, the insulating film 116 is formed over the insulating film 104,the metal oxide 108, and the conductive film 112. By the formation ofthe insulating film 116, part of the metal oxide 108 a that is incontact with the insulating film 116 becomes the regions 108 n. Here,the metal oxide 108 a overlapping with the conductive film 112 is themetal oxide 108 (see FIG. 10C).

The insulating film 116 can be formed using a material selected from theabove-mentioned materials. In this embodiment, as the insulating film116, a 100-nm-thick silicon nitride oxide film is formed with a PECVDapparatus. In the formation of the silicon nitride oxide film, twosteps, i.e., plasma treatment and deposition treatment, are performed ata temperature of 220° C. The plasma treatment is performed under thefollowing conditions: an argon gas at a flow rate of 100 sccm and anitrogen gas at a flow rate of 1000 sccm are introduced into a chamberbefore deposition; the pressure in the chamber is set to 40 Pa; and apower of 1000 W is supplied to an RF power source (27.12 MHz). Thedeposition treatment is performed under the following conditions: asilane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of5000 sccm, and an ammonia gas at a flow rate of 100 sccm are introducedinto the chamber; the pressure in the chamber is set to 100 Pa; and apower of 1000 W is supplied to the RF power source (27.12 MHz).

When a silicon nitride oxide film is used as the insulating film 116,nitrogen or hydrogen in the silicon nitride oxide film can be suppliedto the regions 108 n in contact with the insulating film 116. Inaddition, when the formation temperature of the insulating film 116 isthe above temperature, release of excess oxygen contained in theinsulating film 110 to the outside can be suppressed.

Next, the insulating film 118 is formed over the insulating film 116(see FIG. 11A).

The insulating film 118 can be formed using a material selected from theabove-mentioned materials. In this embodiment, as the insulating film118, a 300-nm-thick silicon oxynitride film is formed with a PECVDapparatus.

Subsequently, a mask is formed by lithography in a desired position overthe insulating film 118, and then the insulating film 118 and theinsulating film 116 are partly etched, so that the opening 141 a and theopening 141 b reaching the regions 108 n are formed (see FIG. 11B).

To etch the insulating film 118 and the insulating film 116, a wetetching method and/or a dry etching method can be used. In thisembodiment, the insulating film 118 and the insulating film 116 areprocessed by a dry etching method.

Next, a conductive film is formed over the regions 108 n and theinsulating film 118 so as to cover the openings 141 a and 141 b, and theconductive film is processed into a desired shape, whereby theconductive films 120 a and 120 b are formed (see FIG. 11C).

The conductive films 120 a and 120 b can be formed using a materialselected from the above-mentioned materials. In this embodiment, for theconductive films 120 a and 120 b, a stack including a 50-nm-thicktungsten film and a 400-nm-thick copper film is formed with a sputteringapparatus.

To process the conductive film to be the conductive films 120 a and 120b, a wet etching method and/or a dry etching method can be used. In thisembodiment, in the processing of the conductive film into the conductivefilms 120 a and 120 b, the copper film is etched by a wet etchingmethod, and then the tungsten film is etched by a dry etching method.

Then, the insulating film 122 is formed to cover the conductive films120 a and 120 b and the insulating film 118.

Through the above steps, the transistor 150 in FIGS. 7A to 7C can bemanufactured.

Note that the films included in the transistor 150 (the insulating film,the metal oxide film, the conductive film, and the like) can be formedby, other than the above methods, a sputtering method, a chemical vapordeposition (CVD) method, a vacuum evaporation method, a pulsed laserdeposition (PLD) method, or an ALD method. Alternatively, a coatingmethod or a printing method can be used. Although a sputtering methodand a PECVD method are typical examples of the deposition method, athermal CVD method may be used. As an example of a thermal CVD method, ametal organic chemical vapor deposition (MOCVD) method can be given.

Deposition by the thermal CVD method may be performed in such a mannerthat the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and a source gas and an oxidizer are supplied to thechamber at a time and react with each other in the vicinity of thesubstrate or over the substrate. Thus, no plasma is generated in thedeposition; therefore, the thermal CVD method has an advantage that nodefect due to plasma damage is caused.

The films such as the conductive films, the insulating films, and themetal oxide films that are described above can be formed by a thermalCVD method such as an MOCVD method.

For example, in the case where a hafnium oxide film is formed with adeposition apparatus employing an ALD method, two kinds of gases areused, namely, ozone (O₃) as an oxidizer and a source gas that isobtained by vaporizing liquid containing a solvent and a hafniumprecursor (hafnium alkoxide or hafnium amide such astetrakis(dimethylamide)hafnium (TDMAH, Hf[N(CH₃)₂]₄) ortetrakis(ethylmethylamide)hafnium).

In the case where an aluminum oxide film is formed with a depositionapparatus employing an ALD method, two kinds of gases are used, namely,H₂O as an oxidizer and a source gas that is obtained by vaporizingliquid containing a solvent and an aluminum precursor (e.g.,trimethylaluminum (TMA, Al(CH₃)₃)). Examples of another material includetris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

In the case where a silicon oxide film is formed with a depositionapparatus employing an ALD method, hexachlorodisilane is adsorbed on asurface on which a film is to be formed, and radicals of an oxidizinggas (O₂ or dinitrogen monoxide) are supplied to react with theadsorbate.

In the case where a tungsten film is formed with a deposition apparatusemploying an ALD method, a WF₆ gas and a B₂H₆ gas are sequentiallyintroduced to form an initial tungsten film, and then, a WF₆ gas and anH₂ gas are used to form a tungsten film. Note that an SiH₄ gas may beused instead of a B₂H₆ gas.

In the case where a metal oxide such as an In—Ga—Zn—O film is formedwith a deposition apparatus employing an ALD method, an In(CH₃)₃ gas andan O₃ gas are used to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gasare used to form a Ga—O layer, and then, a Zn(CH₃)₂ gas and an O₃ gasare used to form a Zn—O layer. Note that the order of these layers isnot limited to this example. A mixed compound layer such as an In—Ga—Olayer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed by using thesegases. Note that although an H₂O gas that is obtained by bubbling waterwith an inert gas such as Ar may be used instead of an O₃ gas, it ispreferable to use an O₃ gas, which does not contain H.

<2-6. Structure Example 4 of Transistor>

FIG. 12A is a top view of a transistor 300A. FIG. 12B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 12A.FIG. 12C is a cross-sectional view taken along dashed-dotted line Y1-Y2in FIG. 12A. Note that in FIG. 12A, some components of the transistor300A (e.g., an insulating film functioning as a gate insulating film)are not illustrated to avoid complexity. The direction of thedashed-dotted line X1-X2 may be referred to as a channel lengthdirection, and the direction of the dashed-dotted line Y1-Y2 may bereferred to as a channel width direction. As in FIG. 12A, somecomponents are not illustrated in some cases in top views of transistorsdescribed below.

The transistor 300A illustrated in FIGS. 12A to 12C includes aconductive film 304 over a substrate 302, an insulating film 306 overthe substrate 302 and the conductive film 304, an insulating film 307over the insulating film 306, a metal oxide 308 over the insulating film307, a conductive film 312 a over the metal oxide 308, and a conductivefilm 312 b over the metal oxide 308. Over the transistor 300A,specifically, over the conductive films 312 a and 312 b and the metaloxide 308, an insulating film 314, an insulating film 316, and aninsulating film 318 are provided.

In the transistor 300A, the insulating films 306 and 307 function as thegate insulating films of the transistor 300A, and the insulating films314, 316, and 318 function as protective insulating films of thetransistor 300A. Furthermore, in the transistor 300A, the conductivefilm 304 functions as a gate electrode, the conductive film 312 afunctions as a source electrode, and the conductive film 312 b functionsas a drain electrode.

In this specification and the like, the insulating films 306 and 307 maybe referred to as a first insulating film, the insulating films 314 and316 may be referred to as a second insulating film, and the insulatingfilm 318 may be referred to as a third insulating film.

The transistor 300A illustrated in FIGS. 12A to 12C is a channel-etchedtransistor. The metal oxide of one embodiment of the present inventionis suitable for a channel-etched transistor.

<2-7. Structure Example 5 of Transistor>

FIG. 13A is a top view of a transistor 300B. FIG. 13B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 13A.FIG. 13C is a cross-sectional view taken along dashed-dotted line Y1-Y2in FIG. 13A.

The transistor 300B illustrated in FIGS. 13A to 13C includes theconductive film 304 over the substrate 302, the insulating film 306 overthe substrate 302 and the conductive film 304, the insulating film 307over the insulating film 306, the metal oxide 308 over the insulatingfilm 307, the insulating film 314 over the metal oxide 308, theinsulating film 316 over the insulating film 314, the conductive film312 a electrically connected to the metal oxide 308 through an opening341 a provided in the insulating films 314 and 316, and the conductivefilm 312 b electrically connected to the metal oxide 308 through anopening 341 b provided in the insulating films 314 and 316. Over thetransistor 300B, specifically, over the conductive films 312 a and 312 band the insulating film 316, the insulating film 318 is provided.

In the transistor 300B, the insulating films 306 and 307 each functionas a gate insulating film of the transistor 300B, the insulating films314 and 316 each function as a protective insulating film of the metaloxide 308, and the insulating film 318 functions as a protectiveinsulating film of the transistor 300B. Moreover, in the transistor300B, the conductive film 304 functions as a gate electrode, theconductive film 312 a functions as a source electrode, and theconductive film 312 b functions as a drain electrode.

The transistor 300A illustrated in FIGS. 12A to 12C has a channel-etchedstructure, whereas the transistor 300B in FIGS. 13A to 13C has achannel-protective structure. The metal oxide of one embodiment of thepresent invention is suitable for a channel-protective transistor aswell.

<2-8. Structure Example 6 of Transistor>

FIG. 14A is a top view of a transistor 300C. FIG. 14B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 14A.FIG. 14C is a cross-sectional view taken along dashed-dotted line Y1-Y2in FIG. 14A.

The transistor 300C illustrated in FIGS. 14A to 14C is different fromthe transistor 300B in FIGS. 13A to 13C in the shapes of the insulatingfilms 314 and 316. Specifically, the insulating films 314 and 316 of thetransistor 300C have island shapes and are provided over a channelregion of the metal oxide 308. Other components are similar to those ofthe transistor 300B.

<2-9. Structure Example 7 of Transistor>

FIG. 15A is a top view of a transistor 300D. FIG. 15B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 15A.FIG. 15C is a cross-sectional view taken along dashed-dotted line Y1-Y2in FIG. 15A.

The transistor 300D illustrated in FIGS. 15A to 15C includes theconductive film 304 over the substrate 302, the insulating film 306 overthe substrate 302 and the conductive film 304, the insulating film 307over the insulating film 306, the metal oxide 308 over the insulatingfilm 307, the conductive film 312 a over the metal oxide 308, theconductive film 312 b over the metal oxide 308, the insulating film 314over the metal oxide 308 and the conductive films 312 a and 312 b, theinsulating film 316 over the insulating film 314, the insulating film318 over the insulating film 316, and conductive films 320 a and 320 bover the insulating film 318.

In the transistor 300D, the insulating films 306 and 307 function asfirst gate insulating films of the transistor 300D, and the insulatingfilms 314, 316, and 318 function as second gate insulating films of thetransistor 300D. Furthermore, in the transistor 300D, the conductivefilm 304 functions as a first gate electrode, the conductive film 320 afunctions as a second gate electrode, and the conductive film 320 bfunctions as a pixel electrode used for a display device. The conductivefilm 312 a functions as a source electrode, and the conductive film 312b functions as a drain electrode.

As illustrated in FIG. 15C, the conductive film 320 b is connected tothe conductive film 304 in an opening 342 b and an opening 342 cprovided in the insulating films 306, 307, 314, 316, and 318. Thus, thesame potential is applied to the conductive film 320 b and theconductive film 304.

The structure of the transistor 300D is not limited to that describedabove, in which the openings 342 b and 342 c are provided so that theconductive film 320 b is connected to the conductive film 304. Forexample, a structure in which only one of the openings 342 b and 342 cis provided so that the conductive film 320 b is connected to theconductive film 304, or a structure in which the conductive film 320 bis not connected to the conductive film 304 without providing theopenings 342 b and 342 c may be employed. Note that in the case wherethe conductive film 320 b is not connected to the conductive film 304,it is possible to apply different potentials to the conductive film 320b and the conductive film 304.

The conductive film 320 b is connected to the conductive film 312 bthrough an opening 342 a provided in the insulating films 314, 316, and318.

Note that the transistor 300D has the S-channel structure describedabove.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 3

In this embodiment, an example of a display panel which can be used fora display portion or the like in a display device including thesemiconductor device of one embodiment of the present invention isdescribed with reference to FIG. 17 and FIG. 18. The display paneldescribed below as an example includes both a reflective liquid crystalelement and a light-emitting element and can display an image in boththe transmissive mode and the reflective mode. Note that the metal oxideof one embodiment of the present invention and a transistor includingthe metal oxide can be preferably used in a transistor in a pixel of adisplay device, a driver for driving the display device, an LSIsupplying data to the display device, or the like.

<Structure Example of Display Panel>

FIG. 17 is a schematic perspective view illustrating a display panel 600of one embodiment of the present invention. In the display panel 600, asubstrate 651 and a substrate 661 are attached to each other. In FIG.17, the substrate 661 is denoted by a dashed line.

The display panel 600 includes a display portion 662, a circuit 659, awiring 666, and the like. The substrate 651 is provided with the circuit659, the wiring 666, a conductive film 663 that serves as a pixelelectrode, and the like. In FIG. 17, an IC 673 and an FPC 672 aremounted on the substrate 651. Thus, the structure illustrated in FIG. 17can be referred to as a display module including the display panel 600,the FPC 672, and the IC 673.

As the circuit 659, for example, a circuit functioning as a scan linedriver circuit can be used.

The wiring 666 has a function of supplying a signal or electric power tothe display portion 662 or the circuit 659. The signal or electric poweris input to the wiring 666 from the outside through the FPC 672 or fromthe IC 673.

FIG. 17 shows an example in which the IC 673 is provided on thesubstrate 651 by a chip on glass (COG) method or the like. As the IC673, an IC functioning as a scan line driver circuit, a signal linedriver circuit, or the like can be used. Note that it is possible thatthe IC 673 is not provided when, for example, the display panel 600includes circuits serving as a scan line driver circuit and a signalline driver circuit and when the circuits serving as a scan line drivercircuit and a signal line driver circuit are provided outside and asignal for driving the display panel 600 is input through the FPC 672.Alternatively, the IC 673 may be mounted on the FPC 672 by a chip onfilm (COF) method or the like.

FIG. 17 also shows an enlarged view of part of the display portion 662.The conductive films 663 included in a plurality of display elements arearranged in a matrix in the display portion 662. The conductive film 663has a function of reflecting visible light and serves as a reflectiveelectrode of a liquid crystal element 640 described later.

As illustrated in FIG. 17, the conductive film 663 has an opening. Alight-emitting element 660 is positioned closer to the substrate 651than the conductive film 663 is. Light is emitted from thelight-emitting element 660 to the substrate 661 side through the openingin the conductive film 663.

<Cross-Sectional Structure Example>

FIG. 18 shows an example of cross sections of part of a region includingthe FPC 672, part of a region including the circuit 659, and part of aregion including the display portion 662 of the display panelillustrated in FIG. 17.

The display panel includes an insulating film 620 between the substrates651 and 661. The display panel also includes the light-emitting element660, a transistor 601, a transistor 605, a transistor 606, a coloringlayer 634, and the like between the substrate 651 and the insulatingfilm 620. Furthermore, the display panel includes the liquid crystalelement 640, a coloring layer 631, and the like between the insulatingfilm 620 and the substrate 661. The substrate 661 and the insulatingfilm 620 are bonded with an adhesive layer 641. The substrate 651 andthe insulating film 620 are bonded with an adhesive layer 642.

The transistor 606 is electrically connected to the liquid crystalelement 640 and the transistor 605 is electrically connected to thelight-emitting element 660. Since the transistors 605 and 606 are formedon a surface of the insulating film 620 which is on the substrate 651side, the transistors 605 and 606 can be formed through the sameprocess.

The substrate 661 is provided with the coloring layer 631, alight-blocking film 632, an insulating film 621, a conductive film 613serving as a common electrode of the liquid crystal element 640, analignment film 633 b, an insulating film 617, and the like. Theinsulating film 617 serves as a spacer for holding a cell gap of theliquid crystal element 640.

Insulating layers such as an insulating film 681, an insulating film682, an insulating film 683, an insulating film 684, and an insulatingfilm 685 are provided on the substrate 651 side of the insulating film620. Part of the insulating film 681 functions as a gate insulatinglayer of each transistor. The insulating films 682, 683, and 684 areprovided to cover each transistor. The insulating film 685 is providedto cover the insulating film 684. The insulating films 684 and 685 eachfunction as a planarization layer. Note that an example where the threeinsulating layers, the insulating films 682, 683, and 684, are providedto cover the transistors and the like is described here; however, oneembodiment of the present invention is not limited to this example, andfour or more insulating layers, a single insulating layer, or twoinsulating layers may be provided. The insulating film 684 functioningas a planarization layer is not necessarily provided when not needed.

The transistors 601, 605, and 606 each include a conductive film 654part of which functions as a gate, a conductive film 652 part of whichfunctions as a source or a drain, and a semiconductor film 653. Here, aplurality of layers obtained by processing the same conductive film areshown with the same hatching pattern.

The liquid crystal element 640 is a reflective liquid crystal element.The liquid crystal element 640 has a stacked structure of a conductivefilm 635, a liquid crystal layer 612, and the conductive film 613. Inaddition, the conductive film 663 which reflects visible light isprovided in contact with the surface of the conductive film 635 thatfaces the substrate 651. The conductive film 663 includes an opening655. The conductive films 635 and 613 contain a material transmittingvisible light. In addition, an alignment film 633 a is provided betweenthe liquid crystal layer 612 and the conductive film 635 and thealignment film 633 b is provided between the liquid crystal layer 612and the conductive film 613. A polarizing plate 656 is provided on anouter surface of the substrate 661.

In the liquid crystal element 640, the conductive film 663 has afunction of reflecting visible light and the conductive film 613 has afunction of transmitting visible light. Light entering from thesubstrate 661 side is polarized by the polarizing plate 656, passesthrough the conductive film 613 and the liquid crystal layer 612, and isreflected by the conductive film 663. Then, the light passes through theliquid crystal layer 612 and the conductive film 613 again and reachesthe polarizing plate 656. In this case, alignment of the liquid crystalis controlled with a voltage that is applied between the conductive film613 and the conductive films 663 and 635, and thus optical modulation oflight can be controlled. That is, the intensity of light emitted throughthe polarizing plate 656 can be controlled. Light excluding light in aparticular wavelength region is absorbed by the coloring layer 631, andthus, emitted light is red light, for example.

The light-emitting element 660 is a bottom-emission light-emittingelement. The light-emitting element 660 has a structure in which aconductive film 643, an EL layer 644, and a conductive film 645 b arestacked in this order from the insulating film 620 side. In addition, aconductive film 645 a is provided to cover the conductive film 645 b.The conductive film 645 b contains a material reflecting visible light,and the conductive films 643 and 645 a contain a material transmittingvisible light. Light is emitted from the light-emitting element 660 tothe substrate 661 side through the coloring layer 634, the insulatingfilm 620, the opening 655, the conductive film 613, and the like.

Here, as illustrated in FIG. 18, the conductive film 635 transmittingvisible light is preferably provided for the opening 655. Accordingly,the liquid crystal is aligned in a region overlapping with the opening655 as well as in the other regions, in which case an alignment defectof the liquid crystal is prevented from being generated in the boundaryportion of these regions and undesired light leakage can be suppressed.

As the polarizing plate 656 provided on an outer surface of thesubstrate 661, a linear polarizing plate or a circularly polarizingplate can be used. An example of a circularly polarizing plate is astack including a linear polarizing plate and a quarter-wave retardationplate. Such a structure can reduce reflection of external light. Thecell gap, alignment, drive voltage, and the like of the liquid crystalelement used as the liquid crystal element 640 are controlled dependingon the kind of the polarizing plate so that desirable contrast isobtained.

In addition, an insulating film 647 is provided on the insulating film646 covering an end portion of the conductive film 643. The insulatingfilm 647 has a function as a spacer for preventing the insulating film620 and the substrate 651 from getting closer more than necessary. Inthe case where the EL layer 644 or the conductive film 645 a is formedusing a blocking mask (metal mask), the insulating film 647 may have afunction of preventing the blocking mask from being in contact with asurface on which the EL layer 644 or the conductive film 645 a isformed. Note that the insulating film 647 is not necessarily providedwhen not needed.

One of a source and a drain of the transistor 605 is electricallyconnected to the conductive film 643 of the light-emitting element 660through a conductive film 648.

One of a source and a drain of the transistor 606 is electricallyconnected to the conductive film 663 through a connection portion 607.The conductive films 663 and 635 are in contact with and electricallyconnected to each other. Here, in the connection portion 607, theconductive layers provided on both surfaces of the insulating film 620are connected to each other through an opening in the insulating film620.

A connection portion 604 is provided in a region where the substrate 651and the substrate 661 do not overlap with each other. The connectionportion 604 is electrically connected to the FPC 672 through aconnection layer 649. The connection portion 604 has a structure similarto that of the connection portion 607. On the top surface of theconnection portion 604, a conductive layer obtained by processing thesame conductive film as the conductive film 635 is exposed. Thus, theconnection portion 604 and the FPC 672 can be electrically connected toeach other through the connection layer 649.

A connection portion 687 is provided in part of a region where theadhesive layer 641 is provided. In the connection portion 687, theconductive layer obtained by processing the same conductive film as theconductive film 635 is electrically connected to part of the conductivefilm 613 with a connector 686. Accordingly, a signal or a potentialinput from the FPC 672 connected to the substrate 651 side can besupplied to the conductive film 613 formed on the substrate 661 sidethrough the connection portion 687.

As the connector 686, a conductive particle can be used, for example. Asthe conductive particle, a particle of an organic resin, silica, or thelike coated with a metal material can be used. It is preferable to usenickel or gold as the metal material because contact resistance can bereduced. It is also preferable to use a particle coated with layers oftwo or more kinds of metal materials, such as a particle coated withnickel and further with gold. As the connector 686, a material capableof elastic deformation or plastic deformation is preferably used. Asillustrated in FIG. 18, the connector 686 which is the conductiveparticle has a shape that is vertically crushed in some cases. With thecrushed shape, the contact area between the connector 686 and aconductive layer electrically connected to the connector 686 can beincreased, thereby reducing contact resistance and suppressing thegeneration of problems such as disconnection.

The connector 686 is preferably provided so as to be covered with theadhesive layer 641. For example, the connectors 686 are dispersed in theadhesive layer 641 before curing of the adhesive layer 641.

FIG. 18 illustrates an example of the circuit 659 in which thetransistor 601 is provided.

The structure in which the semiconductor film 653 where a channel isformed is provided between two gates is used as an example of thetransistors 601 and 605 in FIG. 18. One gate is formed using theconductive film 654 and the other gate is formed using a conductive film623 overlapping with the semiconductor film 653 with the insulating film682 provided therebetween. Such a structure enables control of thresholdvoltages of a transistor. In that case, the two gates may be connectedto each other and supplied with the same signal to operate thetransistor. Such a transistor can have higher field-effect mobility andthus have higher on-state current than other transistors. Consequently,a circuit capable of high-speed operation can be obtained. Furthermore,the area occupied by a circuit portion can be reduced. The use of thetransistor having high on-state current can reduce signal delay inwirings and can reduce display unevenness even in a display panel inwhich the number of wirings is increased because of increase in size orresolution.

Note that the transistor included in the circuit 659 and the transistorincluded in the display portion 662 may have the same structure. Aplurality of transistors included in the circuit 659 may have the samestructure or different structures. A plurality of transistors includedin the display portion 662 may have the same structure or differentstructures.

A material through which impurities such as water and hydrogen do noteasily diffuse is preferably used for at least one of the insulatingfilms 682 and 683 which cover the transistors. That is, the insulatingfilm 682 or the insulating film 683 can function as a barrier film. Sucha structure can effectively suppress diffusion of the impurities intothe transistors from the outside, and a highly reliable display panelcan be provided.

The insulating film 621 is provided on the substrate 661 side to coverthe coloring layer 631 and the light-blocking film 632. The insulatingfilm 621 may have a function as a planarization layer. The insulatingfilm 621 enables the conductive film 613 to have an almost flat surface,resulting in a uniform alignment state of the liquid crystal layer 612.

An example of the method for manufacturing the display panel 600 isdescribed. For example, the conductive film 635, the conductive film663, and the insulating film 620 are formed in this order over a supportsubstrate provided with a separation layer, and the transistor 605, thetransistor 606, the light-emitting element 660, and the like are formed.Then, the substrate 651 and the support substrate are bonded with theadhesive layer 642. After that, separation is performed at the interfacebetween the separation layer and each of the insulating film 620 and theconductive film 635, whereby the support substrate and the separationlayer are removed. Separately, the coloring layer 631, thelight-blocking film 632, the conductive film 613, and the like areformed over the substrate 661 in advance. Then, the liquid crystal isdropped onto the substrate 651 or 661 and the substrates 651 and 661 arebonded with the adhesive layer 641, whereby the display panel 600 can bemanufactured.

A material for the separation layer can be selected such that separationat the interface with the insulating film 620 and the conductive film635 occurs. In particular, it is preferable that a stacked layer of alayer including a high-melting-point metal material, such as tungsten,and a layer including an oxide of the metal material be used as theseparation layer, and a stacked layer of a plurality of layers, such asa silicon nitride layer, a silicon oxynitride layer, and a siliconnitride oxide layer be used as the insulating film 620 over theseparation layer. The use of the high-melting-point metal material forthe separation layer can increase the formation temperature of a layerformed in a later step, which reduces impurity concentration andachieves a highly reliable display panel.

As the conductive film 635, an oxide or a nitride such as a metal oxideor a metal nitride is preferably used. In the case of using a metaloxide, a material in which at least one of the concentrations ofhydrogen, boron, phosphorus, nitrogen, and other impurities and thenumber of oxygen vacancies is made to be higher than those in asemiconductor layer of a transistor is used for the conductive film 635.

<Components>

The above components will be described below. Note that descriptions ofstructures having functions similar to those in the above embodimentsare omitted.

[Adhesive Layer]

As the adhesive layer, a variety of curable adhesives such as a reactivecurable adhesive, a thermosetting adhesive, an anaerobic adhesive, and aphotocurable adhesive such as an ultraviolet curable adhesive can beused. Examples of these adhesives include an epoxy resin, an acrylicresin, a silicone resin, a phenol resin, a polyimide resin, an imideresin, a polyvinyl chloride (PVC) resin, a polyvinyl butyral (PVB)resin, and an ethylene vinyl acetate (EVA) resin. In particular, amaterial with low moisture permeability, such as an epoxy resin, ispreferred. Alternatively, a two-component-mixture-type resin may beused. Further alternatively, an adhesive sheet or the like may be used.

Furthermore, the resin may include a drying agent. For example, asubstance that adsorbs moisture by chemical adsorption, such as an oxideof an alkaline earth metal (e.g., calcium oxide or barium oxide), can beused. Alternatively, a substance that adsorbs moisture by physicaladsorption, such as zeolite or silica gel, may be used. The drying agentis preferably included because it can prevent impurities such asmoisture from entering the element, thereby improving the reliability ofthe display panel.

In addition, it is preferable to mix a filler with a high refractiveindex or a light-scattering member into the resin, in which case lightextraction efficiency can be enhanced. For example, titanium oxide,barium oxide, zeolite, zirconium, or the like can be used.

[Connection Layer]

As the connection layer, an anisotropic conductive film (ACF), ananisotropic conductive paste (ACP), or the like can be used.

[Coloring Layer]

Examples of a material that can be used for the coloring layers includea metal material, a resin material, and a resin material containing apigment or dye.

[Light-Blocking Layer]

Examples of a material that can be used for the light-blocking layerinclude carbon black, titanium black, a metal, a metal oxide, and acomposite oxide containing a solid solution of a plurality of metaloxides. The light-blocking layer may be a film containing a resinmaterial or a thin film of an inorganic material such as a metal.Stacked films containing the material of the coloring layer can also beused for the light-blocking layer. For example, a stacked-layerstructure of a film containing a material of a coloring layer whichtransmits light of a certain color and a film containing a material of acoloring layer which transmits light of another color can be employed.It is preferable that the coloring layer and the light-blocking layer beformed using the same material because the same manufacturing apparatuscan be used and the process can be simplified.

The above is the description of the components.

<Manufacturing Method Example>

A manufacturing method example of a display panel using a flexiblesubstrate is described.

Here, layers including a display element, a circuit, a wiring, anelectrode, optical members such as a coloring layer and a light-blockinglayer, an insulating layer, and the like, are collectively referred toas an element layer. The element layer includes, for example, a displayelement, and may additionally include a wiring electrically connected tothe display element or an element such as a transistor used in a pixelor a circuit.

In addition, here, a flexible member which supports the element layer ata stage at which the display element is completed (the manufacturingprocess is finished) is referred to as a substrate. For example, asubstrate includes an extremely thin film with a thickness greater thanor equal to 10 nm and less than or equal to 300 μm and the like.

As a method for forming an element layer over a flexible substrateprovided with an insulating surface, typically, there are two methodsshown below. One of them is to directly form an element layer over thesubstrate. The other method is to form an element layer over a supportsubstrate that is different from the substrate and then to separate theelement layer from the support substrate to be transferred to thesubstrate. Although not described in detail here, in addition to theabove two methods, there is a method in which an element layer is formedover a substrate which does not have flexibility and the substrate isthinned by polishing or the like to have flexibility.

In the case where a material of the substrate can withstand heatingtemperature in a process for forming the element layer, it is preferablethat the element layer be formed directly over the substrate, in whichcase a manufacturing process can be simplified. At this time, theelement layer is preferably formed in a state where the substrate isfixed to the support substrate, in which case transfer thereof in anapparatus and between apparatuses can be easy.

In the case of employing the method in which the element layer is formedover the support substrate and then transferred to the substrate, first,a separation layer and an insulating layer are stacked over the supportsubstrate, and then the element layer is formed over the insulatinglayer. Next, the element layer is separated from the support substrateand then transferred to the substrate. At this time, selected is amaterial with which separation at an interface between the supportsubstrate and the separation layer, at an interface between theseparation layer and the insulating layer, or in the separation layeroccurs. With the method, it is preferable that a material having highheat resistance be used for the support substrate or the separationlayer, in which case the upper limit of the temperature applied when theelement layer is formed can be increased, and an element layer includinga more highly reliable element can be formed.

For example, it is preferable that a stack of a layer containing ahigh-melting-point metal material, such as tungsten, and a layercontaining an oxide of the metal material be used as the separationlayer, and a stack of a plurality of layers, such as a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, and asilicon nitride oxide layer be used as the insulating layer over theseparation layer.

As the method for separating the support substrate from the elementlayer, applying mechanical force, etching the separation layer, andmaking a liquid permeate the separation interface are given as examples.Alternatively, separation may be performed by heating or cooling twolayers of the separation interface by utilizing a difference in thermalexpansion coefficient.

The separation layer is not necessarily provided in the case where theseparation can be performed at an interface between the supportsubstrate and the insulating layer.

For example, glass and an organic resin such as polyimide can be used asthe support substrate and the insulating layer, respectively. In thatcase, a separation trigger may be formed by, for example, locallyheating part of the organic resin with laser light or the like, or byphysically cutting part of or making a hole through the organic resinwith a sharp tool, and separation may be performed at an interfacebetween the glass and the organic resin. As the above-described organicresin, a photosensitive material is preferably used because an openingor the like can be easily formed. The above-described laser lightpreferably has a wavelength region, for example, from visible light toultraviolet light. For example, light having a wavelength of greaterthan or equal to 200 nm and less than or equal to 400 nm, preferablygreater than or equal to 250 nm and less than or equal to 350 nm can beused. In particular, an excimer laser having a wavelength of 308 nm ispreferably used because the productivity is increased. Alternatively, asolid-state UV laser (also referred to as a semiconductor UV laser),such as a UV laser having a wavelength of 355 nm which is the thirdharmonic of an Nd:YAG laser, may be used.

Alternatively, a heat generation layer may be provided between thesupport substrate and the insulating layer formed of an organic resin,and separation may be performed at an interface between the heatgeneration layer and the insulating layer by heating the heat generationlayer. For the heat generation layer, any of a variety of materials suchas a material which generates heat by feeding current, a material whichgenerates heat by absorbing light, and a material which generates heatby applying a magnetic field can be used. For example, for the heatgeneration layer, a material selected from a semiconductor, a metal, andan insulator can be used.

In the above-described methods, the insulating layer formed of anorganic resin can be used as a substrate after the separation.

The above is the description of a manufacturing method of a flexibledisplay panel.

At least part of this embodiment can be implemented in combination withany of the other embodiments described in this specification asappropriate.

Embodiment 4

In this embodiment, a metal oxide of one embodiment of the presentinvention is described.

The metal oxide of one embodiment of the present invention includesindium (In), M (M is Al, Ga, Y, or Sn), and zinc (Zn). Specifically, Mis preferably gallium (Ga). In the following description, Ga is used asM.

Here, the case where silicon (Si), boron (B), or carbon (C) exists as animpurity in an In—Ga—Zn oxide is described.

<Calculation Model and Calculation Method>

First, calculations were performed using a reference model of anIn—Ga—Zn oxide in an amorphous state which has no impurities, a model inwhich one Si atom is added to the reference model, a model in which oneB atom is added to the reference model, and a model in which one C atomis added to the reference model.

Specifically, a model 700 with [In]:[Ga]:[Zn]:[O]=1:1:1:4 shown in FIG.19A was used as the reference crystal model. Note that the model 700includes 112 atoms.

Strictly, an In-M-Zn oxide having a CAC composition is not in anamorphous state. On the other hand, the In-M-Zn oxide having the CACcomposition has lower crystallinity than an In-M-Zn oxide having a CAACstructure. Accordingly, a model in an amorphous state was used forconvenience to reduce influence of the crystal structure and observe thebonding state.

In the model 700, a Si atom, a B atom, or a C atom was assumed to existas an impurity, and one Si atom, one B atom, or one C atom was locatedin an interstitial site of the model 700. Note that one impurity wasadded to the model 700 including the 112 atoms. Accordingly, theimpurity concentration of the model is approximately 7×10²⁰/cm³.

In the case where one Si atom exists as an impurity, a local structure702 which is the vicinity of the Si atom extracted from a model in whichthe Si atom is bonded to four O atoms is shown in FIG. 20A, and a localstructure 704 which is the vicinity of the Si atom extracted from amodel in which the Si atom is bonded to three O atoms and one Ga atom isshown in FIG. 20C.

In the case where one B atom exists as an impurity, a local structure706 which is the vicinity of the B atom extracted from a model in whichthe B atom is bonded to three O atoms is shown in FIG. 21A, and a localstructure 708 which is the vicinity of the B atom extracted from themodel is shown in FIG. 21C.

In the case where one C atom exists as an impurity, a local structure710 which is the vicinity of the C atom extracted from a model in whichthe C atom is bonded to two O atoms and one Ga atom is shown in FIG.22A, and a local structure 712 which is the vicinity of the C atomextracted from a model in which the C atom is bonded to one O atom andone Ga atom is shown in FIG. 22C.

The specific calculation is as follows. A first principle electronicstate calculation package, Vienna Ab initio Simulation Package (VASP),was used for the atomic relaxation calculation. The calculationconditions are listed in Table 1 below.

TABLE 1 Software VASP Exchange-correlation functional GGA-PBEPseudopotential PAW method Cut-off energy of plane wave 800 ev Samplingpoint k Structure optimization 1 × 1 × 1 Density of states 2 × 2 × 2<Density of States>

FIG. 19B shows the density of states in FIG. 19A. In FIG. 19B, the Fermilevel (the energy of the highest occupied level of electrons) isadjusted to be 0 eV in the horizontal axis. It is found from FIG. 19Bthat the electrons reach the valence band maximum and the level in thegap does not exist.

FIGS. 20B and 20D show the density of states in the case where one Siatom is added as an impurity. Note that FIG. 20B shows the density ofstates in the case where the local structure 702 in FIG. 20A isincluded. FIG. 20D shows the density of states in the case where thelocal structure 704 in FIG. 20C is included.

It is found from FIGS. 20B and 20D that when a Si atom exists, the Fermilevel is located in the conduction band. This indicates that carriersare generated in an In—Ga—Zn oxide (the In—Ga—Zn oxide is made to be ann-type) owing to Si atoms.

FIGS. 21B and 21D show the density of states in the case where one Batom is added as an impurity. Note that FIG. 21B shows the density ofstates in the case where the local structure 706 in FIG. 21A isincluded. FIG. 21D shows the density of states in the case where thelocal structure 708 in FIG. 21C is included.

It was found from FIGS. 21B and 21D that when a B atom exists, the Fermilevel is located in the conduction band. This indicates that carriersare generated in an In—Ga—Zn oxide (the In—Ga—Zn oxide is made to be ann-type) owing to B atoms.

FIGS. 22B and 22D show the density of states in the case where one Catom is added as an impurity. Note that FIG. 22B shows the density ofstates in the case where the local structure 710 in FIG. 22A isincluded. FIG. 22D shows the density of states in the case where thelocal structure 712 in FIG. 22C is included.

It was found from FIGS. 22B and 22D that when a C atom exists, the Fermilevel is located in the conduction band. This indicates that carriersare generated in an In—Ga—Zn oxide (the In—Ga—Zn oxide is made to be ann-type) owing to C atoms.

It is highly probable that Si atoms and B atoms exist as cations in anIn—Ga—Zn oxide because the electronegativities of Si and B are closer tothe electronegativities of In, Ga, and Zn than the electronegativity ofO. Thus, it is supposed that carriers are generated.

Although C is bonded to a metal and O because the electronegativity of Cis between the electronegativity of O and the electronegativities of In,Ga, and Zn, it is assumed that C is likely to exist as cationsbasically.

Furthermore, a Si atom, a B atom, and a C atom are more strongly bondedto an O atom than an In atom, a Ga atom, and a Zn atom are. For thatreason, by the entry of a Si atom, a B atom, and a C atom, O atomsbonded to an In atom, a Ga atom, and a Zn atom are trapped by the Siatom, the B atom, and the C atom, which probably forms deep levelscorresponding to oxygen vacancies.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Example 1

In this example, measurement results of a metal oxide of one embodimentof the present invention over a substrate are described. A variety ofmethods were used for the measurement. Note that in this example,Samples 1A, 1B, 1C, 1D, 1E, 1F, 1G, 1H, and 1J were fabricated.

<<Structure of Samples and Fabrication Method Thereof>>

Samples 1A to 1H and 1J relating to one embodiment of the presentinvention are described below. Samples 1A to 1H and 1J each include asubstrate and a metal oxide over the substrate.

Samples 1A to 1H and 1J were fabricated at different temperatures anddifferent oxygen flow rate ratios in formation of the metal oxide. Thetemperatures and the oxygen flow rate ratios in formation of the metaloxides of Samples 1A to 1H and 1J are shown in Table 2 below.

TABLE 2 Flow rate Formation [sccm] O₂ ratio temperature O₂ Ar [%] [° C.]Sample 1A 30 270 10 R.T. Sample 1B 90 210 30 R.T. Sample 1C 300 0 100R.T. Sample 1D 30 270 10 130 Sample 1E 90 210 30 130 Sample 1F 300 0 100130 Sample 1G 30 270 10 170 Sample 1H 90 210 30 170 Sample 1J 300 0 100170

Next, methods for fabricating the samples will be described.

A glass substrate was used as the substrate. Over the substrate, a100-nm-thick In—Ga—Zn oxide was formed as a metal oxide with asputtering apparatus. The formation conditions were as follows: thepressure in a chamber was 0.6 Pa; and a metal oxide target (an atomicratio of In:Ga:Zn=4:2:4.1) was used as a target. The metal oxide targetprovided in the sputtering apparatus was supplied with an AC power of2500 W.

The formation temperatures and oxygen flow rate ratios shown in theabove table were used as the conditions for forming metal oxides tofabricate Samples 1A to 1H and 1J.

Through the above steps, Samples 1A to 1H and 1J of this example werefabricated.

<Analysis by X-Ray Diffraction>

In this section, results of X-ray diffraction (XRD) measurementperformed on the metal oxides over the glass substrates are described.As an XRD apparatus, D8 ADVANCE manufactured by Bruker AXS was used. Theconditions were as follows: scanning was performed by an out-of-planemethod at θ/2θ, the scanning range was 15 deg. to 50 deg.; the stepwidth was 0.02 deg.; and the scanning speed was 3.0 deg./min.

FIG. 23 shows an XRD spectrum of the samples measured by an out-of-planemethod.

In the XRD spectra shown in FIG. 23, the higher the substratetemperature at the time of formation is or the higher the oxygen gasflow rate ratio at the time of formation is, the higher the intensity ofthe peak at around 2θ=31° is. Note that it is found that the peak ataround 2θ=31° is derived from a crystalline IGZO compound whose c-axesare aligned in a direction substantially perpendicular to a formationsurface or a top surface of the crystalline IGZO compound (such acompound is also referred to as c-axis aligned crystalline (CAAC) IGZO).

As shown in the XRD spectra in FIG. 23, as the substrate temperature atthe time of formation is lower or the oxygen gas flow rate ratio at thetime of formation is lower, a peak becomes less clear. Accordingly, itis found that there are no alignment in the a-b plane direction andc-axis alignment in the measured areas of the samples that are formed ata lower substrate temperature or with a lower oxygen gas flow rateratio.

<TEM Images and Electron Diffraction>

This section describes the observation and analysis results of Samples1A, 1D, and 1J with a high-angle annular dark-field scanningtransmission electron microscope (HAADF-STEM). An image obtained with anHAADF-STEM is also referred to as a TEM image.

This section describes electron diffraction patterns obtained byirradiation of Samples 1A, 1D, and 1J with an electron beam with a probediameter of 1 nm (also referred to as a nanobeam).

The plan-view TEM images were observed with a spherical aberrationcorrector function. The HAADF-STEM images were obtained using an atomicresolution analytical electron microscope JEM-ARM200F manufactured byJEOL Ltd. under the following conditions: the acceleration voltage was200 kV; and irradiation with an electron beam with a diameter ofapproximately 0.1 nm was performed.

Note that the electron diffraction patterns were observed while anelectron beam irradiation was performed in the following manner that theelectron beam was moved at a constant rate for 35 seconds.

FIG. 24A shows a cross-sectional TEM image of Sample 1A, and FIG. 24Bshows an electron diffraction pattern of Sample 1A. FIG. 24C shows across-sectional TEM image of Sample 1D, and FIG. 24D shows an electrondiffraction pattern of Sample 1D. FIG. 24E shows a cross-sectional TEMimage of Sample 1J, and FIG. 24F shows an electron diffraction patternof Sample 1J.

It is known that, for example, when an electron beam with a probediameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄crystal in a direction parallel to the sample surface, a diffractionpattern including a spot derived from the (009) plane of the InGaZnO₄crystal is obtained. That is, the CAAC-OS has c-axis alignment and thec-axes are aligned in the direction substantially perpendicular to theformation surface or the top surface of the CAAC-OS. Meanwhile, aring-like diffraction pattern is shown when an electron beam with aprobe diameter of 300 nm is incident on the same sample in a directionperpendicular to the sample surface. That is, it is found that theCAAC-OS has neither a-axis alignment nor b-axis alignment.

Furthermore, a diffraction pattern like a halo pattern is observed whena metal oxide including a nanocrystal (in particular, in the case wheresuch a metal oxide has a function similar to that of a semiconductor, itis referred to as a nanocrystalline oxide semiconductor (nc-OS)) issubjected to electron diffraction using an electron beam with a largeprobe diameter (e.g., 50 nm or larger). Meanwhile, bright spots areshown in a nanobeam electron diffraction pattern of the metal oxideincluding a nanocrystal obtained using an electron beam with a smallprobe diameter (e.g., smaller than 50 nm). Furthermore, in a nanobeamelectron diffraction pattern of the metal oxide including a nanocrystal,regions with high luminance in a circular (ring) pattern are shown insome cases. Also in a nanobeam electron diffraction pattern of the metaloxide including a nanocrystal, a plurality of bright spots are shown ina ring-like shape in some cases.

A nanocrystal (hereinafter, also referred to as nc) is found in Sample1A from the result of the cross-sectional TEM observation in FIG. 24A.As shown in FIG. 24B, the observed electron diffraction pattern ofSample 1A has a region with high luminance in a circular (ring) pattern.Furthermore, a plurality of spots are shown in the ring-shaped region.

Sample 1D is found to have a CAAC structure and a nanocrystal from theresult of the cross-sectional TEM observation in FIG. 24C. As shown inFIG. 24D, the observed electron diffraction pattern of Sample 1D has aregion with high luminance in a circular (ring) pattern. Furthermore, aplurality of spots are shown in the ring-shaped region. In thediffraction pattern, spots derived from the (009) plane are slightlyobserved.

Sample 1J is clearly found to have layered arrangement of a CAACstructure from the result of the cross-sectional TEM observation in FIG.24E. Furthermore, spots derived from the (009) plane are clearlyobserved from the result of the electron diffraction pattern of Sample1J in FIG. 24F.

The features observed in the cross-sectional TEM images and theplan-view TEM images are one aspect of a structure of a metal oxide.

Next, electron diffraction patterns obtained by irradiation of Sample 1Awith an electron beam with a probe diameter of 1 nm (also referred to asa nanobeam) are shown in FIGS. 25A to 25L.

Electron diffraction patterns of points indicated by black dots a1, a2,a3, a4, and a5 in the plan-view TEM image of Sample 1A in FIG. 25A areobserved. Note that the electron diffraction patterns are observed whileelectron beam irradiation is performed in the following manner that theelectron beam was moved at a constant rate for 35 seconds. FIGS. 25C,25D, 25E, 25F, and 25G show the results of the points indicated by theblack dots a1, a2, a3, a4, and a5, respectively.

In FIGS. 25C, 25D, 25E, 25F, and 25G, regions with high luminance in aring pattern were shown. Furthermore, a plurality of spots are shown inthe ring-shaped regions.

Electron diffraction patterns of points indicated by black dots b1, b2,b3, b4, and b5 in the cross-sectional TEM image of Sample 1A in FIG. 25Bare observed. FIGS. 25H, 25I, 25J, 25K, and 25L show the results of thepoints indicated by the black dots b1, b2, b3, b4, and b5, respectively.

In FIGS. 25H, 25I, 25J, 25K, and 25L, regions with high luminance in aring pattern are shown. Furthermore, a plurality of spots are shown inthe ring-shaped regions.

That is, it is found that Sample 1A has an nc structure and hascharacteristics distinctly different from those of a metal oxide havingan amorphous structure and those of a metal oxide having a singlecrystal structure.

According to the above description, the electron diffraction patterns ofSample 1A and Sample 1D each have a region with high luminance in a ringpattern and a plurality of bright spots appear in the ring-shapedregion. Accordingly, Sample 1A exhibits an electron diffraction patternof the metal oxide including nanocrystals and does not show alignment inthe plane direction and the cross-sectional direction. Sample 1D isfound to be a mixed material of the nc structure and the CAAC structure.

In the electron diffraction pattern of Sample 1J, spots derived from the(009) plane of an InGaZnO₄ crystal are included. Thus, Sample 1J hasc-axis alignment and the c-axes are aligned in the directionsubstantially perpendicular to the formation surface or the top surfaceof Sample 1J.

<Analysis of TEM Image>

This section describes the observation and analysis results of Samples1A, 1C, 1D, 1F, and 1G with an HAADF-STEM.

The results of image analysis of plan-view TEM images are described. Theplan-view TEM images were obtained with a spherical aberration correctorfunction. The plan-view TEM images were obtained using an atomicresolution analytical electron microscope JEM-ARM200F manufactured byJEOL Ltd. under the following conditions: the acceleration voltage was200 kV; and irradiation with an electron beam with a diameter ofapproximately 0.1 nm was performed.

In FIG. 26, the plan-view TEM images of Samples 1A, 1C, 1D, 1F, 1G, and1J and images obtained through image processing of the plan-view TEMimages are shown. Note that in a table in FIG. 26, left views are theplan-view TEM images and right views are the images obtained throughimage processing of the plan-view TEM images on the left side.

Image processing and image analyzing methods are described. Imageprocessing was performed as follows. The plan-view TEM image in FIG. 26was subjected to fast Fourier transform (FFT), so that an FFT image wasobtained. Then, the obtained FFT image was subjected to mask processingexcept for a range from 2.8 nm⁻¹ to 5.0 nm⁻¹. After that, the FFT imagesubjected to mask processing was subjected to inverse fast Fouriertransform (IFFT) to obtain an FFT filtering image.

To conduct the image analysis, lattice points were extracted from theFFT filtering image in the following manner. First, noise in the FFTfiltering image was removed. To remove the noise, the luminance of aregion within a 0.05-nm radius was smoothed using Formula 1.

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\mspace{616mu}} & \; \\{{{S\_ Int}\mspace{11mu}\left( {x,y} \right)} = {\sum\limits_{r \leq 0.05}\frac{{Int}\mspace{11mu}\left( {x^{\prime},y^{\prime}} \right)}{r}}} & (1)\end{matrix}$

Note that S_Int(x,y) represents the smoothed luminance at thecoordinates (x,y), r represents the distance between the coordinates(x,y) and the coordinates (x′,y′), and Int(x′,y′) represents theluminance at the coordinates (x′,y′). In the calculation, r is regardedas 1 when it is 0.

Then, a search for lattice points was conducted. The coordinates withthe highest luminance among candidate lattice points within a 0.22-nmradius were regarded as the lattice point. At this point, a candidatelattice point was extracted. Within a 0.22-nm radius, detection errorsof lattice points due to noise can be less frequent. Note that adjacentlattice points are a certain distance away from each other in the TEMimage; thus, two or more lattice points are unlikely to be observedwithin a 0.22-nm radius.

Subsequently, coordinates with the highest luminance within a 0.22-nmradius from the extracted candidate lattice point were extracted toredetermine a candidate lattice point. The extraction of a candidatelattice point was repeated in this manner until no new candidate latticepoint appeared; the coordinates at that point were determined as alattice point. Similarly, determination of another lattice point wasperformed at a position 0.22 nm or more away from the determined latticepoint; thus, lattice points were determined in the entire region. Thedetermined lattice points are collectively called a lattice point group.

Here, a method for deriving an orientation of a hexagonal lattice fromthe extracted lattice point group is described with reference toschematic diagrams in FIGS. 27A to 27C and a flow chart in FIG. 27D.First, a reference lattice point was determined and the six closestlattice points to the reference lattice point were connected to form ahexagonal lattice (see FIG. 27A and Step S101 in FIG. 27D). After that,an average distance R between the reference lattice point, which was thecenter point of the hexagonal lattice, and each of the lattice points,which is a vertex, was calculated. Then, a regular hexagon was formedwith the use of the reference lattice point as the center point and thecalculated distance R as the distance from the center point to eachvertex (see Step S102 in FIG. 27D). The distances from the vertices ofthe regular hexagon to their respective closest lattice points wereregarded as a distance d1, a distance d2, a distance d3, a distance d4,a distance d5, and a distance d6 (see FIG. 27B and Step S103 in FIG.27D). Next, the regular hexagon was rotated around the center pointthrough 60° by 0.1°, and the average deviation between the hexagonallattice and the rotated regular hexagon [D=(d1+d2+d3+d4+d5+d6)/6] wascalculated (see Step S104 in FIG. 27D). Then, a rotation angle θ of theregular hexagon when the average deviation D becomes minimum wascalculated as the orientation of the hexagonal lattice (see FIG. 27C andStep S105 in FIG. 27D).

Next, an observation area of the plan-view TEM image was adjusted sothat hexagonal lattices whose orientations were 30° account for thehighest percentage. In such a condition, the average orientation ofhexagonal lattice within a 1-nm radius was calculated. The plan-view TEMimage obtained through image processing was shown where color orgradation changes in accordance with the angle of the hexagonal latticein the region. The image obtained through image processing of theplan-view TEM image in FIG. 26 is an image obtained by performing imageanalysis on the plan-view TEM image in FIG. 26 by the above method andapplying color in accordance with the angle of the hexagonal lattice. Inother words, the image obtained through the image processing of theplan-view TEM image is an image in which the orientations of latticepoints in certain wavenumber ranges are extracted by color-coding thecertain wavenumber ranges in an FFT filtering image of the plan-view TEMimage.

As shown in FIG. 26, in Samples 1A and 1D in which nc is observed, thehexagons are oriented randomly and distributed in a mosaic pattern. InSample 1J in which a layered structure is observed in thecross-sectional TEM image, regions with uniformly oriented hexagonsexist in a large area of several tens of nanometers. In Sample 1D, it isfound that an nc region in a random mosaic pattern and a large-arearegion with uniformly oriented hexagons as in Sample 1J are included.

It is found from FIG. 26 that as the substrate temperature at the timeof formation is lower or the oxygen gas flow rate ratio at the time offormation is lower, regions in which the hexagons are oriented randomlyand distributed in a mosaic pattern are likely to exist.

Through the analysis of a plan-view TEM image of a CAAC-OS, a boundaryportion where angles of hexagonal lattices change can be examined.

Next, Voronoi diagrams were formed using lattice point groups in Sample1A. A Voronoi diagram is an image partitioned by regions including alattice point group. Each lattice point is closer to regions surroundingthe lattice point than to any other lattice point. A method for forminga Voronoi diagram is described below in detail using schematic diagramsin FIGS. 28A to 28D and a flow chart in FIG. 28E.

First, a lattice point group was extracted by the method described usingFIGS. 27A to 27D or the like (see FIG. 28A and Step S111 in FIG. 28E).Next, adjacent lattice points were connected with segments (see FIG. 28Band Step S112 in FIG. 28E). Then, perpendicular bisectors of thesegments were drawn (see FIG. 28C and Step S113 in FIG. 28E).Subsequently, points where three perpendicular bisectors intersect wereextracted (see Step S114 in FIG. 28E). The points are called Voronoipoints. After that, adjacent Voronoi points were connected with segments(see FIG. 28D and Step S115 in FIG. 28E). A polygonal region surroundedby the segments at this point is called a Voronoi region. In the abovemethod, a Voronoi diagram was formed.

FIG. 29 shows the proportions of the shapes of Voronoi regions(tetragon, pentagon, hexagon, heptagon, octagon, and enneagon) inSamples 1A, 1C, 1D, 1F, 1G, and 1J. Bar graphs show the numbers of theshapes of Voronoi regions (tetragon, pentagon, hexagon, heptagon,octagon, and enneagon) in the samples. Furthermore, tables show theproportions of the shapes of Voronoi regions (tetragon, pentagon,hexagon, heptagon, octagon, and enneagon) in the samples.

It is found from FIG. 29 that there is a tendency that the proportion ofhexagons is high in Sample 1J with a high degree of crystallinity andthe proportion of hexagons is low in Sample 1A with a low degree ofcrystallinity. The proportion of hexagons in Sample 1D is between thosein Samples 1J and 1A. Accordingly, it is found from FIG. 29 that thecrystal state of the metal oxide significantly differs under differentformation conditions.

It is found from FIG. 29 that as the substrate temperature at the timeof formation is lower or the oxygen gas flow rate ratio at the time offormation is lower, the degree of crystallinity is lower and theproportion of hexagons is lower.

<Elementary Analysis>

This section describes the analysis results of elements included inSample 1A. For the analysis, by energy dispersive X-ray spectroscopy(EDX), EDX mapping images are obtained. An energy dispersive X-rayspectrometer AnalysisStation JED-2300T manufactured by JEOL Ltd. is usedas an elementary analysis apparatus in the EDX measurement. A Si driftdetector is used to detect an X-ray emitted from the sample.

In the EDX measurement, an EDX spectrum of a point is obtained in such amanner that electron beam irradiation is performed on the point in adetection target region of a sample, and the energy of characteristicX-ray of the sample generated by the irradiation and its frequency aremeasured. In this example, peaks of an EDX spectrum of the point wereattributed to electron transition to the L shell in an In atom, electrontransition to the K shell in a Ga atom, electron transition to the Kshell in a Zn atom, and electron transition to the K shell in an O atom,and the proportions of the atoms in the point are calculated. An EDXmapping image indicating distributions of proportions of atoms can beobtained through the process in an analysis target region of a sample.

FIGS. 30A to 30H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1A. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 30B to 30D and 30F to30H is 7,200,000 times.

FIG. 30A shows a cross-sectional TEM image, and FIG. 30E shows aplan-view TEM image. FIG. 30B shows a cross-sectional EDX mapping imageof In atoms, and FIG. 30F shows a plan-view EDX mapping image of Inatoms. In the EDX mapping image in FIG. 30B, the proportion of the Inatoms in all the atoms is 9.28 atomic % to 33.74 atomic %. In the EDXmapping image in FIG. 30F, the proportion of the In atoms in all theatoms is 12.97 atomic % to 38.01 atomic %.

FIG. 30C shows a cross-sectional EDX mapping image of Ga atoms, and FIG.30G shows a plan-view EDX mapping image of Ga atoms. In the EDX mappingimage in FIG. 30C, the proportion of the Ga atoms in all the atoms is1.18 atomic % to 18.64 atomic %. In the EDX mapping image in FIG. 30G,the proportion of the Ga atoms in all the atoms is 1.72 atomic % to19.82 atomic %.

FIG. 30D shows a cross-sectional EDX mapping image of Zn atoms, and FIG.30H shows a plan-view EDX mapping image of Zn atoms. In the EDX mappingimage in FIG. 30D, the proportion of the Zn atoms in all the atoms is6.69 atomic % to 24.99 atomic %. In the EDX mapping image in FIG. 30H,the proportion of the Zn atoms in all the atoms is 9.29 atomic % to28.32 atomic %.

Note that FIGS. 30A to 30D show the same region in the cross section ofSample 1A. FIGS. 30E to 30H show the same region in the plane of Sample1A.

FIGS. 31A to 31F show enlarged cross-sectional EDX mapping images andenlarged plan-view EDX mapping images of Sample 1A. FIG. 31A is anenlarged view of a part in FIG. 30B. FIG. 31B is an enlarged view of apart in FIG. 30C. FIG. 31C is an enlarged view of a part in FIG. 30D.FIG. 31D is an enlarged view of a part in FIG. 30F. FIG. 31E is anenlarged view of a part in FIG. 30G. FIG. 31F is an enlarged view of apart in FIG. 30H.

The EDX mapping images in FIGS. 31A to 31C show relative distribution ofbright and dark areas, indicating that the atoms have distributions inSample 1A. Areas surrounded by solid lines and areas surrounded bydashed lines in FIGS. 31A to 31C are examined.

As shown in FIG. 31A, a relatively bright region occupies a large areain the area surrounded by the solid line and a relatively dark regionoccupies in a large area in the area surrounded by the dashed line. Asshown in FIG. 31B, a relatively dark region occupies a large area in thearea surrounded by the solid line and a relatively bright regionoccupies a large area in the area surrounded by the dashed line.

That is, it is found that the areas surrounded by the solid lines areregions including a relatively large number of In atoms and the areassurrounded by the dashed lines are regions including a relatively smallnumber of In atoms. FIG. 31C shows that a lower portion of the areasurrounded by the solid line is relatively bright and an upper portionthereof is relatively dark. Thus, it is found that the area surroundedby the solid line is a region including In_(X2)Zn_(Y2)O_(Z2), InO_(X1),or the like as a main component.

It is found that the area surrounded by the solid line is a regionincluding a relatively small number of Ga atoms and the area surroundedby the dashed line is a region including a relatively large number of Gaatoms. FIG. 31C shows that a left portion of the area surrounded by thedashed line is relatively dark and a right portion thereof is relativelybright. Thus, it is found that the area surrounded by the dashed line isa region including GaO_(X3), Ga_(X4)Zn_(Y4)O_(Z4), or the like as a maincomponent.

Similarly, areas surrounded by solid lines and dashed lines in the EDXmapping images in FIGS. 31D to 31F are examined.

As shown in FIG. 31D, a relatively bright region occupies a large areain the area surrounded by the solid line and a relatively dark regionoccupies a large area in the area surrounded by the dashed line. Asshown in FIG. 31E, a relatively dark region occupies a large area in thearea surrounded by the solid line and a relatively bright regionoccupies a large area in the area surrounded by the dashed line.

That is, it is found that the areas surrounded by the solid lines areregions including a relatively large number of In atoms and a relativelysmall number of Ga atoms. FIG. 31F shows that a lower portion of thearea surrounded by the solid line is relatively dark and an upperportion thereof is relatively bright. Thus, it is found that the areasurrounded by the solid line is a region including In_(X2)Zn_(Y2)O_(Z2),InO_(X1), or the like as a main component.

It is found that the area surrounded by the dashed line is a regionincluding a relatively small number of In atoms and a region including arelatively large number of Ga atoms. FIG. 31F shows that a right portionof the area surrounded by the dashed line is relatively dark and a leftportion thereof is relatively bright. Thus, it is found that the areasurrounded by the dashed line is a region including GaO_(X3),Ga_(X4)Zn_(Y4)O_(Z4), or the like as a main component.

Furthermore, as shown in FIGS. 31A to 31F, the In atoms are relativelymore uniformly distributed than the Ga atoms, and regions includingInO_(X1) as a main component are seemingly joined to each other througha region including In_(X2)Zn_(Y2)O_(Z2) as a main component. It can bethus guessed that the regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component extend like a cloud.

An In—Ga—Zn oxide having a composition in which the regions includingGaO_(X3) as a main component and the regions includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component are unevenlydistributed and mixed can be referred to as CAC-IGZO.

As shown in FIGS. 31A to 31F, each of the regions including GaO_(X3) asa main component and the regions including In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component has a size of greater than or equal to 0.5nm and less than or equal to 10 nm, or greater than or equal to 1 nm andless than or equal to 3 nm.

As described above, it is confirmed that the CAC-IGZO has a structuredifferent from that of an IGZO compound in which metal elements areevenly distributed, and has characteristics different from those of theIGZO compound. That is, it can be confirmed that in the CAC-IGZO,regions including GaO_(X3) or the like as a main component and regionsincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component areseparated to form a mosaic pattern.

Accordingly, it can be expected that when CAC-IGZO is used for asemiconductor element, the property derived from GaO_(X3) or the likeand the property derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)complement each other, whereby high on-state current (I_(on)), highfield-effect mobility (μ), and low off-state current (I_(off)) can beachieved. A semiconductor element including CAC-IGZO has highreliability. Thus, CAC-IGZO is suitably used in a variety ofsemiconductor devices typified by a display.

[Samples 1B to 1J]

FIGS. 31-2A to 31-2H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1B. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-2B to 31-2D and31-2F to 31-2H is 7,200,000 times.

FIG. 31-2A shows a cross-sectional TEM image, and FIG. 31-2E shows aplan-view TEM image. FIG. 31-2B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-2F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-2B, the proportion of theIn atoms in all the atoms is 12.60 atomic % to 40.60 atomic %. In theEDX mapping image in FIG. 31-2F, the proportion of the In atoms in allthe atoms is 11.94 atomic % to 39.97 atomic %.

FIG. 31-2C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-2G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-2C, the proportion of the Ga atoms in all theatoms is 2.30 atomic % to 23.73 atomic %. In the EDX mapping image inFIG. 31-2G, the proportion of the Ga atoms in all the atoms is 0.00atomic % to 20.45 atomic %.

FIG. 31-2D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-2H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-2D, the proportion of the Zn atoms in all theatoms is 9.85 atomic % to 33.11 atomic %. In the EDX mapping image inFIG. 31-2H, the proportion of the Zn atoms in all the atoms is 7.49atomic % to 27.69 atomic %.

Note that FIGS. 31-2A to 31-2D show the same region in the cross sectionof Sample 1B. FIGS. 31-2E to 31-2H show the same region in the plane ofSample 1B.

FIGS. 31-3A to 31-3H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1C. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-3B to 31-3D and31-3F to 31-3H is 7,200,000 times.

FIG. 31-3A shows a cross-sectional TEM image, and FIG. 31-3E shows aplan-view TEM image. FIG. 31-3B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-3F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-3B, the proportion of theIn atoms in all the atoms is 14.79 atomic % to 39.71 atomic %. In theEDX mapping image in FIG. 31-3F, the proportion of the In atoms in allthe atoms is 14.08 atomic % to 50.62 atomic %.

FIG. 31-3C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-3G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-3C, the proportion of the Ga atoms in all theatoms is 2.83 atomic % to 21.61 atomic %. In the EDX mapping image inFIG. 31-3G, the proportion of the Ga atoms in all the atoms is 0.00atomic % to 21.13 atomic %.

FIG. 31-3D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-3H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-3D, the proportion of the Zn atoms in all theatoms is 10.27 atomic % to 28.84 atomic %. In the EDX mapping image inFIG. 31-3H, the proportion of the Zn atoms in all the atoms is 3.64atomic % to 32.84 atomic %.

Note that FIGS. 31-3A to 31-3D show the same region in the cross sectionof Sample 1C. FIGS. 31-3E to 31-3H show the same region in the plane ofSample 1C.

FIGS. 31-4A to 31-4H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1D. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-4B to 31-4D and31-4F to 31-4H is 7,200,000 times.

FIG. 31-4A shows a cross-sectional TEM image, and FIG. 31-4E shows aplan-view TEM image. FIG. 31-4B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-4F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-4B, the proportion of theIn atoms in all the atoms is 9.61 atomic % to 40.10 atomic %. In the EDXmapping image in FIG. 31-4F, the proportion of the In atoms in all theatoms is 13.83 atomic to 45.10 atomic %.

FIG. 31-4C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-4G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-4C, the proportion of the Ga atoms in all theatoms is 0.00 atomic % to 21.14 atomic %. In the EDX mapping image inFIG. 31-4G, the proportion of the Ga atoms in all the atoms is 1.28atomic % to 23.63 atomic %.

FIG. 31-4D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-4H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-4D, the proportion of the Zn atoms in all theatoms is 6.13 atomic % to 28.45 atomic %. In the EDX mapping image inFIG. 31-4H, the proportion of the Zn atoms in all the atoms is 7.50atomic % to 31.90 atomic %.

Note that FIGS. 31-4A to 31-4D show the same region in the cross sectionof Sample 1D. FIGS. 31-4E to 31-4H show the same region in the plane ofSample 1D.

FIGS. 31-5A to 31-5H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1E. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-5B to 31-5D and31-5F to 31-5H is 7,200,000 times.

FIG. 31-5A shows a cross-sectional TEM image, and FIG. 31-5E shows aplan-view TEM image. FIG. 31-5B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-5F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-5B, the proportion of theIn atoms in all the atoms is 10.35 atomic % to 40.04 atomic %. In theEDX mapping image in FIG. 31-5F, the proportion of the In atoms in allthe atoms is 6.90 atomic % to 42.28 atomic %.

FIG. 31-5C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-5G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-5C, the proportion of the Ga atoms in all theatoms is 0.00 atomic % to 23.03 atomic %. In the EDX mapping image inFIG. 31-5G, the proportion of the Ga atoms in all the atoms is 0.00atomic % to 22.85 atomic %.

FIG. 31-5D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-5H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-5D, the proportion of the Zn atoms in all theatoms is 5.10 atomic % to 30.74 atomic %. In the EDX mapping image inFIG. 31-5H, the proportion of the Zn atoms in all the atoms is 0.63atomic % to 31.76 atomic %.

Note that FIGS. 31-5A to 31-5D show the same region in the cross sectionof Sample 1E. FIGS. 31-5E to 31-5H show the same region in the plane ofSample 1E.

FIGS. 31-6A to 31-6H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1F. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-6B to 31-6D and31-6F to 31-6H is 7,200,000 times.

FIG. 31-6A shows a cross-sectional TEM image, and FIG. 31-6E shows aplan-view TEM image. FIG. 31-6B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-6F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-6B, the proportion of theIn atoms in all the atoms is 13.37 atomic % to 42.99 atomic %. In theEDX mapping image in FIG. 31-6F, the proportion of the In atoms in allthe atoms is 8.73 atomic % to 46.45 atomic %.

FIG. 31-6C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-6G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-6C, the proportion of the Ga atoms in all theatoms is 2.56 atomic % to 26.02 atomic %. In the EDX mapping image inFIG. 31-6G, the proportion of the Ga atoms in all the atoms is 0.00atomic % to 22.51 atomic %.

FIG. 31-6D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-6H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-6D, the proportion of the Zn atoms in all theatoms is 8.66 atomic % to 32.64 atomic %. In the EDX mapping image inFIG. 31-6H, the proportion of the Zn atoms in all the atoms is 1.41atomic % to 29.64 atomic %.

Note that FIGS. 31-6A to 31-6D show the same region in the cross sectionof Sample 1F. FIGS. 31-6E to 31-6H show the same region in the plane ofSample 1F.

FIGS. 31-7A to 31-7H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1G. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-7B to 31-7D and31-7F to 31-7H is 7,200,000 times.

FIG. 31-7A shows a cross-sectional TEM image, and FIG. 31-7E shows aplan-view TEM image. FIG. 31-7B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-7F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-7B, the proportion of theIn atoms in all the atoms is 10.76 atomic % to 43.30 atomic %. In theEDX mapping image in FIG. 31-7F, the proportion of the In atoms in allthe atoms is 6.93 atomic % to 43.39 atomic %.

FIG. 31-7C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-7G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-7C, the proportion of the Ga atoms in all theatoms is 1.40 atomic % to 23.07 atomic %. In the EDX mapping image inFIG. 31-7G, the proportion of the Ga atoms in all the atoms is 0.00atomic % to 21.30 atomic %.

FIG. 31-7D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-7H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-7D, the proportion of the Zn atoms in all theatoms is 4.67 atomic % to 28.72 atomic %. In the EDX mapping image inFIG. 31-7H, the proportion of the Zn atoms in all the atoms is 4.16atomic % to 30.54 atomic %.

Note that FIGS. 31-7A to 31-7D show the same region in the cross sectionof Sample 1G. FIGS. 31-7E to 31-7H show the same region in the plane ofSample 1G.

FIGS. 31-8A to 31-8H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1H. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-8B to 31-8D and31-8F to 31-8H is 7,200,000 times.

FIG. 31-8A shows a cross-sectional TEM image, and FIG. 31-8E shows aplan-view TEM image. FIG. 31-8B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-8F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-8B, the proportion of theIn atoms in all the atoms is 12.67 atomic % to 43.09 atomic %. In theEDX mapping image in FIG. 31-8F, the proportion of the In atoms in allthe atoms is 13.99 atomic % to 41.90 atomic %.

FIG. 31-8C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-8G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-8C, the proportion of the Ga atoms in all theatoms is 0.60 atomic % to 27.37 atomic %. In the EDX mapping image inFIG. 31-8G, the proportion of the Ga atoms in all the atoms is 1.67atomic % to 21.53 atomic %.

FIG. 31-8D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-8H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-8D, the proportion of the Zn atoms in all theatoms is 7.85 atomic % to 33.61 atomic %. In the EDX mapping image inFIG. 31-8H, the proportion of the Zn atoms in all the atoms is 5.10atomic % to 30.22 atomic %.

Note that FIGS. 31-8A to 31-8D show the same region in the cross sectionof Sample 1H. FIGS. 31-8E to 31-8H show the same region in the plane ofSample 1H.

FIGS. 31-9A to 31-9H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 1J. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 31-9B to 31-9D and31-9F to 31-9H is 7,200,000 times.

FIG. 31-9A shows a cross-sectional TEM image, and FIG. 31-9E shows aplan-view TEM image. FIG. 31-9B shows a cross-sectional EDX mappingimage of In atoms, and FIG. 31-9F shows a plan-view EDX mapping image ofIn atoms. In the EDX mapping image in FIG. 31-9B, the proportion of theIn atoms in all the atoms is 15.80 atomic % to 40.53 atomic %. In theEDX mapping image in FIG. 31-9F, the proportion of the In atoms in allthe atoms is 16.09 atomic % to 40.33 atomic %.

FIG. 31-9C shows a cross-sectional EDX mapping image of Ga atoms, andFIG. 31-9G shows a plan-view EDX mapping image of Ga atoms. In the EDXmapping image in FIG. 31-9C, the proportion of the Ga atoms in all theatoms is 4.13 atomic % to 22.21 atomic %. In the EDX mapping image inFIG. 31-9G, the proportion of the Ga atoms in all the atoms is 2.42atomic % to 21.37 atomic %.

FIG. 31-9D shows a cross-sectional EDX mapping image of Zn atoms, andFIG. 31-9H shows a plan-view EDX mapping image of Zn atoms. In the EDXmapping image in FIG. 31-9D, the proportion of the Zn atoms in all theatoms is 7.75 atomic % to 26.35 atomic %. In the EDX mapping image inFIG. 31-9H, the proportion of the Zn atoms in all the atoms is 7.88atomic % to 28.36 atomic %.

Note that FIGS. 31-9A to 31-9D show the same region in the cross sectionof Sample 1J. FIGS. 31-9E to 31-9H show the same region in the plane ofSample 1J.

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

Example 2

In this example, the transistor 150 including the metal oxide 108 of oneembodiment of the present invention was fabricated and subjected totests for electrical characteristics and reliability. In this example,nine transistors, i.e., Samples 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, and 2J,were fabricated as the transistor 150 including the metal oxide 108.

<Structure of Samples and Fabrication Method Thereof>

Samples 2A to 2H and 2J relating to one embodiment of the presentinvention are described below. As Samples 2A to 2H and 2J, thetransistors 150 having the structure illustrated in FIGS. 6A to 6C werefabricated by the fabrication method described in Embodiment 2 withreference to FIGS. 9A to 9D, FIGS. 10A to 10C, and FIGS. 11A to 11C.

Samples 2A to 2H and 2J were fabricated at different temperatures anddifferent oxygen flow rate ratios in formation of the metal oxide 108.The temperatures and the oxygen flow rate ratios in formation of themetal oxides of Samples 2A to 2H and 2J are shown in Table 3 below.

TABLE 3 Formation conditions of metal oxide 108 Flow rate Formation[sccm] O₂ ratio temperature O₂ Ar [%] [° C.] Sample 2A 30 270 10 R.T.Sample 2B 90 210 30 R.T. Sample 2C 300 0 100 R.T. Sample 2D 30 270 10130 Sample 2E 90 210 30 130 Sample 2F 300 0 100 130 Sample 2G 30 270 10170 Sample 2H 90 210 30 170 Sample 2J 300 0 100 170

The samples were fabricated by the fabrication method described inEmbodiment 2. The metal oxide 108 was formed using a metal oxide target(In:Ga:Zn=4:2:4.1 [atomic ratio]).

The transistor 150 had a channel length of 2 μm and a channel width of 3μm (hereinafter, also referred to as L/W=2/3 μm) or a channel length of2 μm and a channel width of 50 μm (hereinafter, also referred to asL/W=2/50 μm).

<Electrical Characteristics of Transistors>

Next, I_(d)-V_(g) characteristics of the transistors (L/W=2/3 μm) inSamples 2A to 2H and 2J were measured. As conditions for measuring theI_(d)-V_(g) characteristics of each transistor, a voltage applied to theconductive film 112 serving as a first gate electrode (hereinafter thevoltage is also referred to as gate voltage (V_(g))) and a voltageapplied to the conductive film 106 serving as a second gate electrode(hereinafter the voltage is also referred to as back gate voltage(V_(bg))) were changed from −10 V to +10 V in increments of 0.25 V. Avoltage applied to the conductive film 120 a serving as a sourceelectrode (the voltage is also referred to as source voltage (V_(s)))was 0 V (comm), and a voltage applied to the conductive film 120 bserving as a drain electrode (the voltage is also referred to as drainvoltage (V_(d))) was 0.1 V and 20 V.

[I_(d)-V_(g) Characteristics of Transistor]

Drain current-gate voltage characteristics (I_(d)-V_(g) characteristics)of a transistor are described. FIG. 34A illustrates an example ofI_(d)-V_(g) characteristics of the transistor. FIG. 34A shows the casewhere polycrystalline silicon is used for an active layer of thetransistor for easy understanding. In FIG. 34A, the vertical axis andthe horizontal axis represent I_(d) and V_(g), respectively.

As illustrated in FIG. 34A, I_(d)-V_(g) characteristics are broadlydivided into three regions. A first region, a second region, and a thirdregion are referred to as an off region (OFF region), a subthresholdregion, and an on region (ON region), respectively. A gate voltage at aboundary between the subthreshold region and the on region is referredto as a threshold voltage (V_(th)).

To obtain favorable characteristics of the transistor, it is preferablethat the drain current in the off region (also referred to as off-statecurrent or I_(off)) be low and the drain current in the on region (alsoreferred to as on-state current or I_(on)) be high. As an index of theon-state current of the transistor, the field-effect mobility is oftenused. The details of the field-effect mobility are described later.

To drive the transistor at a low voltage, the slope of the I_(d)-V_(g)characteristics in the subthreshold region is preferably steep. An indexof the degree of change in the I_(d)-V_(g) characteristics in thesubthreshold region is referred to as subthreshold swing (SS) or an Svalue. The S value is represented by the following formula (2).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\mspace{616mu}} & \; \\{{SS} = {\min\mspace{11mu}\left( \frac{\partial V_{g}}{\partial{\log_{10}\left( I_{d} \right)}} \right)}} & (2)\end{matrix}$

The S value is a minimum value of the amount of change in gate voltagewhich is needed for changing a drain current by an order of magnitude inthe subthreshold region. As the S value is smaller, switching operationbetween on and off states can be performed rapidly.

[I_(d)-V_(d) Characteristics of Transistor]

Next, drain current-drain voltage characteristics (I_(d)-V_(d)characteristics) of a transistor are described. FIG. 34B illustrates anexample of I_(d)-V_(d) characteristics of the transistor. In FIG. 34B,the vertical axis and the horizontal axis represent I_(d) and V_(d),respectively.

As illustrated in FIG. 34B, the on region is further divided into tworegions. A first region and a second region are referred to as a linearregion and a saturation region, respectively. In the linear region,drain current increases in a parabola shape in accordance with theincrease in drain voltage. On the other hand, in the saturation region,drain current does not greatly change even when drain voltage changes.According to a vacuum tube, the linear region and the saturation regionare referred to as a triode region and a pentode region, respectively,in some cases.

The linear region indicates the state where V_(g) is higher than V_(d)(V_(d)<V_(g)) in some cases. The saturation region indicates the statewhere V_(d) is higher than V_(g) (V_(g)<V_(d)) in some cases. However,in practice, the threshold voltage of the transistor needs to beconsidered. Thus, the state where a value obtained by subtracting thethreshold voltage of the transistor from the gate voltage is higher thanthe drain voltage (V_(d)<V_(g)−V_(th)) is referred to as the linearregion in some cases. Similarly, the state where a value obtained bysubtracting the threshold voltage of the transistor from the gatevoltage is lower than the drain voltage (V_(g)−V_(th)<V_(d)) is referredto as the saturation region in some cases.

The I_(d)-V_(d) characteristics of the transistor with which current inthe saturation region is constant are expressed as “favorablesaturation” in some cases. The favorable saturation of the transistor isimportant particularly when the transistor is used in an organic ELdisplay. For example, the use of a transistor with favorable saturationas a transistor of a pixel of an organic EL display can suppress achange in luminance of the pixel even when the drain voltage is changed.

[Analysis Model of Drain Current]

Next, an analysis model of the drain current is described. As theanalysis model of the drain current, analytic formulae of drain currentbased on gradual channel approximation (GCA) is known. On the basis ofGCA, the drain current of the transistor is represented by the followingformula (3).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\mspace{616mu}} & \; \\{I_{d} = \left\{ \begin{matrix}{\mu\frac{W}{L}{C_{ox}\left\lbrack {{\left( {{Vg} - V_{th}} \right)V_{d}} - {\frac{1}{2}V_{d}^{2}}} \right\rbrack}{\cdots\left( {{V_{g} - V_{th}} > V_{d}} \right)}} \\{\mu\frac{W}{2L}{C_{ox}\left( {{Vg} - V_{th}} \right)}^{2}{\cdots\left( {{V_{g} - V_{th}} \leq V_{d}} \right)}}\end{matrix} \right.} & (3)\end{matrix}$

In the formula (3), the upper formula is a formula for drain current ina linear region and the lower formula is a formula for drain current ina saturation region. In the formula (3), I_(d) represents the draincurrent, μ represents the mobility of the active layer, L represents thechannel length of the transistor, W represents the channel width of thetransistor, Cox represents the gate capacitance, V_(g) represents thegate voltage, V_(d) represents the drain voltage, and V_(th) representsthe threshold voltage of the transistor.

[Field-Effect Mobility]

Next, field-effect mobility is described. As an index of current drivecapability of a transistor, the field-effect mobility is used. Asdescribed above, the on region of the transistor is divided into thelinear region and the saturation region. From the characteristics in theregions, the field-effect mobility of the transistor can be calculatedon the basis of the analytic formulae of the drain current based on GCA.The field-effect mobility in the linear region and the field-effectmobility in the saturation region are referred to as linear mobility andsaturation mobility, respectively, when they need to be distinguishedfrom each other. The linear mobility is represented by the followingformula (4) and the saturation mobility is represented by the followingformula (5).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\mspace{625mu}} & \; \\{\mu_{FE}^{lin} = {\frac{L}{{WC}_{ox}}\frac{\partial I_{d}}{\partial V_{g}}\frac{1}{V_{d}}}} & (4) \\{\left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\mspace{625mu}} & \; \\{\mu_{FE}^{sat} = {\frac{2L}{{WC}_{ox}}\left( \frac{\partial\sqrt{I_{d}}}{\partial V_{g}} \right)^{2}}} & (5)\end{matrix}$

In this specification and the like, curves calculated from the formula(4) and the formula (5) are referred to as mobility curves. FIG. 35shows mobility curves calculated from the analytic formulae of draincurrent based on GCA. In FIG. 35, the I_(d)-V_(g) characteristics atV_(d)=10 V and the mobility curves of the linear mobility and thesaturation mobility at the time when GCA is assumed to be effective areshown together.

In FIG. 35, the I_(d)-V_(g) characteristics are calculated from theanalytic formulae of drain current based on GCA. The shapes of themobility curves can be a lead to understanding the state of the insideof the transistor.

In FIG. 32, the results of I_(d)-V_(g) characteristics and field-effectmobilities of Samples 2A to 2H and 2J are shown. The solid line and thedashed-dotted line represent I_(d) at V_(d)=20 V and I_(d) at V_(d)=0.1V, respectively. The dashed line represents field-effect mobility. InFIG. 32, the first vertical axis represents I_(d) [A], the secondvertical axis represents field-effect mobility (μ FE) [cm²/Vs], and thehorizontal axis represents V_(g) [V]. The field-effect mobility wascalculated from the value measured at V_(d)=20 V.

As shown in FIG. 32, it is found that Samples 2A to 2H and 2J havedifferent on-state currents (I_(on)) and different field effectmobilities, particularly different field effect mobilities in saturationregions. In particular, the maximum saturation mobilities and the risingcharacteristics of the field-effect mobilities around 0 V differdistinctly.

It is found from FIG. 32 that as the substrate temperature at the timeof formation is lower or the oxygen flow rate ratio at the time offormation is lower, the on-state current (I_(on)) becomes higher and thefield effect mobility rises more steeply around 0 V. In particular,Sample 2A has a maximum field-effect mobility close to 70 cm²/Vs.

<Gate Bias-Temperature Stress Test (GBT Test)>

Next, the reliability of the transistors (L/W=2/50 μm) of Samples 2A to2H and 2J was evaluated. As the reliability evaluation, GBT tests wereused.

The conditions for GBT tests in this example were as follows. A voltageapplied to the conductive film 112 serving as the first gate electrodeand the conductive film 106 serving as the second gate electrode(hereinafter referred to as gate voltage (V_(g))) was ±30 V, and avoltage applied to the conductive film 120 a serving as the sourceelectrode and the conductive film 120 b serving as a drain electrode(hereinafter referred to as source voltage (V_(s)) and drain voltage(V_(d)), respectively) was 0 V (COMMON). The stress temperature was 60°C., the time for stress application was 1 hour, and two kinds ofmeasurement environments, a dark environment and a photo environment(irradiation with light at approximately 10,000 lx from a white LED),were employed.

In other words, the conductive film 120 a serving as the sourceelectrode of the transistor 150 and the conductive film 120 b serving asthe drain electrode of the transistor 150 were set at the samepotential, and a potential different from that of the conductive film120 a serving as the source electrode and the conductive film 120 bserving as the drain electrode was applied to the conductive film 112serving as the first gate electrode and the conductive film 106 servingas the second gate electrode for a certain time (here, one hour).

A case where the potential applied to the conductive film 112 serving asthe first gate electrode and the conductive film 106 serving as thesecond gate electrode is higher than the potential applied to theconductive film 120 a serving as the source electrode and the conductivefilm 120 b serving as the drain electrode is called positive stress, anda case where the potential applied to the conductive film 112 serving asthe first gate electrode and the conductive film 106 serving as thesecond gate electrode is lower than the potential applied to theconductive film 120 a serving as the source electrode and the conductivefilm 120 b serving as the drain electrode is called negative stress.Thus, the reliability evaluation was performed under four conditions intotal, i.e., positive GBT (dark), negative GBT (dark), positive GBT(light irradiation), and negative GBT (light irradiation).

Note that the positive GBT (dark) can be referred to as positive biastemperature stress (PBTS), the negative GBT (dark) as negative biastemperature stress (NBTS), the positive GBT (light irradiation) aspositive bias illumination temperature stress (PBITS), and the negativeGBT (light irradiation) as negative bias illumination temperature stress(NBITS).

FIG. 33 shows the GBT test results of Samples 2A to 2H and 2J. In FIG.33, the vertical axis represents the amount of shift in the thresholdvoltage (ΔV_(th)) of the transistors.

The results in FIG. 33 indicate that the amount of shift in thethreshold voltage (ΔV_(th)) of each of the transistors included inSamples 2A to 2H and 2J was within ±3 V in the GBT tests. Thus, it isconfirmed that the transistors included in Samples 2A to 2H and 2J eachhave high reliability.

Thus, even the IGZO film having low crystallinity is presumed to have alow density of defect states like an IGZO film having highcrystallinity.

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

Example 3

In this example, measurement results of a metal oxide of one embodimentof the present invention formed over a substrate are described. Avariety of methods were used for the measurement. Note that in thisexample, Samples 3A, 3D, and 3J were fabricated.

<<Structure of Samples and Fabrication Method Thereof>>

Samples 3A, 3D, and 3J relating to one embodiment of the presentinvention are described below. Samples 3A, 3D, and 3J each include asubstrate and a metal oxide over the substrate.

Samples 3A, 3D, and 3J were fabricated at different temperatures anddifferent oxygen flow rate ratios in formation of the metal oxide. Thetemperatures and the oxygen flow rate ratios in formation of the metaloxides of Samples 3A, 3D, and 3J are shown in Table 4 below.

TABLE 4 Flow rate Formation [sccm] O₂ ratio temperature O₂ Ar [%] [° C.]Sample 3A 30 270 10 R.T. Sample 3D 30 270 10 130 Sample 3J 150 150 50170

Next, methods for fabricating the samples will be described.

A glass substrate was used as the substrate. Over the substrate, a100-nm-thick In—Ga—Zn metal oxide was formed as a metal oxide with asputtering apparatus. The formation conditions were as follows: thepressure in a chamber was 0.6 Pa, and a metal oxide target (an atomicratio of In:Ga:Zn=1:1:1.2) was used as a target. The metal oxide targetprovided in the sputtering apparatus was supplied with an AC power of2500 W.

The formation temperatures and oxygen flow rate ratios shown in theabove table were used as the conditions for forming metal oxides tofabricate Samples 3A, 3D, and 3J.

Through the above steps, Samples 3A, 3D, and 3J of this example werefabricated.

<TEM Images and Electron Diffraction>

This section describes the TEM observation and analysis results ofSamples 3A, 3D, and 3J.

This section describes electron diffraction patterns obtained byirradiation of Samples 3A, 3D, and 3J with an electron beam with a probediameter of 1 nm (also referred to as a nanobeam).

The plan-view TEM images were observed with a spherical aberrationcorrector function. The HAADF-STEM images were obtained using an atomicresolution analytical electron microscope JEM-ARM200F manufactured byJEOL Ltd. under the following conditions: the acceleration voltage was200 kV, and irradiation with an electron beam with a diameter ofapproximately 0.1 nm was performed.

Note that the electron diffraction patterns were observed while anelectron beam irradiation was performed in the following manner that theelectron beam was moved at a constant rate for 35 seconds.

FIG. 36A shows a cross-sectional TEM image of Sample 3A, and FIG. 36Bshows an electron diffraction pattern of Sample 3A. FIG. 36C shows across-sectional TEM image of Sample 3D, and FIG. 36D shows an electrondiffraction pattern of Sample 3D. FIG. 36E shows a cross-sectional TEMimage of Sample 3J, and FIG. 36F shows an electron diffraction patternof Sample 3J.

A nanocrystal is found in Sample 3A from the result of thecross-sectional TEM observation in FIG. 36A. As shown in FIG. 36B, theobserved electron diffraction pattern of Sample 3A has a region withhigh luminance in a circular (ring) pattern. Furthermore, a plurality ofspots are shown in the ring-shaped region.

Sample 3D is found to have a CAAC structure and a nanocrystal From theresult of the cross-sectional TEM observation in FIG. 36C. As shown inFIG. 36D, the observed electron diffraction pattern of Sample 3D has aregion with high luminance in a circular (ring) pattern. Furthermore, aplurality of spots are shown in the ring-shaped region. In thediffraction pattern, spots derived from the (009) plane are slightlyobserved.

Sample 3J is clearly found to have layered arrangement of a CAACstructure from the result of the cross-sectional TEM observation in FIG.36E. Furthermore, spots derived from the (009) plane are included in theelectron diffraction pattern of Sample 3J in FIG. 36F.

The features observed in the cross-sectional TEM images and theplan-view TEM images are one aspect of a structure of a metal oxide.

According to the above description, the electron diffraction patterns ofSample 3A and Sample 3D each have a region with high luminance in a ringpattern and a plurality of bright spots appear in the ring-shapedregion. Accordingly, Samples 3A and 3D each exhibit an electrondiffraction pattern of the metal oxide including nanocrystals and do notshow alignment in the plane direction and the cross-sectional direction.Sample 3D is found to be a mixed material of the nc structure and theCAAC structure.

In the electron diffraction pattern of Sample 3J, spots derived from the(009) plane of an InGaZnO₄ crystal are included. Thus, Sample 3J hasc-axis alignment and the c-axes are aligned in the directionsubstantially perpendicular to the formation surface or the top surfaceof Sample 3J.

<Analysis of TEM Image>

This section describes the observation and analysis results of Samples3A, 3D, and 3J with an HAADF-STEM.

The results of image analysis of plan-view TEM images are described. Theplan-view TEM images were obtained with a spherical aberration correctorfunction. The plan-view TEM images were obtained using an atomicresolution analytical electron microscope JEM-ARM200F manufactured byJEOL Ltd. under the following conditions: the acceleration voltage was200 kV, and irradiation with an electron beam with a diameter ofapproximately 0.1 nm was performed.

FIG. 37A shows a plan-view TEM image of Sample 3A, and FIG. 37B shows animage obtained through image processing of the plan-view TEM image ofSample 3A. FIG. 37C shows a plan-view TEM image of Sample 3D and FIG.37D shows an image obtained through image processing of the plan-viewTEM image of Sample 3D. FIG. 37E shows a plan-view TEM image of Sample3J and FIG. 37F shows an image obtained through image processing of theplan-view TEM image of Sample 3J.

The images obtained through image processing of the plan-view TEM imagesin FIGS. 37B, 37D, and 37F are images obtained through image analysis ofthe plan-view TEM images in FIGS. 37A, 37C, and 37E by the methoddescribed in Example 1 and applying color in accordance with the angleof the hexagonal lattice. In other words, the images obtained throughthe image processing of the plan-view TEM images are each an image inwhich the orientations of lattice points in certain wavenumber rangesare extracted by color-coding the certain wavenumber ranges in an FFTfiltering image of the plan-view TEM image.

As shown in FIGS. 37A to 37F, in Samples 3A and 3D in which nc isobserved, the hexagons are oriented randomly and distributed in a mosaicpattern. In Sample 3J in which a layered structure is observed in thecross-sectional TEM image, regions with uniformly oriented hexagonsexist in a large area of several tens of nanometers. In Sample 3D, it isfound that an nc region in a random mosaic pattern and a large-arearegion with uniformly oriented hexagons as in Sample 3J are included.

It is found from FIGS. 37A to 37F that as the substrate temperature atthe time of formation is lower or the oxygen gas flow rate ratio at thetime of formation is lower, regions in which the hexagons are orientedrandomly and distributed in a mosaic pattern are likely to exist.

Through the analysis of a plan-view TEM image of a CAAC-OS, a boundaryportion where angles of hexagonal lattices change can be examined.

Next, Voronoi diagrams were formed using lattice point groups in Sample3A. The Voronoi diagrams were obtained by the method described inExample 1.

FIGS. 38A to 38C show the proportions of the shapes of Voronoi regions(tetragon, pentagon, hexagon, heptagon, octagon, and enneagon) inSamples 3A, 3D, and 3J, respectively. Bar graphs show the numbers of theshapes of Voronoi regions (tetragon, pentagon, hexagon, heptagon,octagon, and enneagon) in the samples. Furthermore, tables show theproportions of the shapes of Voronoi regions (tetragon, pentagon,hexagon, heptagon, octagon, and enneagon) in the samples.

It is found from FIGS. 38A to 38C that there is a tendency that theproportion of hexagons is high in Sample 3J with a high degree ofcrystallinity and the proportion of hexagons is low in Sample 3A with alow degree of crystallinity. The proportion of hexagons in Sample 3D isbetween those in Samples 3J and 3A. Accordingly, it is found from FIGS.38A to 38C that the crystal state of the metal oxide significantlydiffers under different formation conditions.

It is found from FIGS. 38A to 38C that as the substrate temperature atthe time of formation is lower or the oxygen gas flow rate ratio at thetime of formation is lower, the degree of crystallinity is lower and theproportion of hexagons is lower.

<Elementary Analysis>

This section describes the analysis results of elements included inSample 3A. For the analysis, by energy dispersive X-ray spectroscopy(EDX), EDX mapping images are obtained. An energy dispersive X-rayspectrometer AnalysisStation JED-2300T manufactured by JEOL Ltd. is usedas an elementary analysis apparatus in the EDX measurement. A Si driftdetector is used to detect an X-ray emitted from the sample.

In the EDX measurement, an EDX spectrum of a point is obtained in such amanner that electron beam irradiation is performed on the point in adetection target region of a sample, and the energy of characteristicX-ray of the sample generated by the irradiation and its frequency aremeasured. In this example, peaks of an EDX spectrum of the point wereattributed to electron transition to the L shell in an In atom, electrontransition to the K shell in a Ga atom, and electron transition to the Kshell in a Zn atom and the K shell in an O atom, and the proportions ofthe atoms in the point are calculated. An EDX mapping image indicatingdistributions of proportions of atoms can be obtained through theprocess in an analysis target region of a sample.

FIGS. 39A to 39H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 3A. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 39B to 39D and 39F to39H is 7,200,000 times.

FIG. 39A shows a cross-sectional TEM image, and FIG. 39E shows aplan-view TEM image. FIG. 39B shows a cross-sectional EDX mapping imageof In atoms, and FIG. 39F shows a plan-view EDX mapping image of Inatoms. In the EDX mapping image in FIG. 39B, the proportion of the Inatoms in all the atoms is 8.64 atomic % to 34.91 atomic %. In the EDXmapping image in FIG. 39F, the proportion of the In atoms in all theatoms is 5.76 atomic % to 34.69 atomic %.

FIG. 39C shows a cross-sectional EDX mapping image of Ga atoms, and FIG.39G shows a plan-view EDX mapping image of Ga atoms. In the EDX mappingimage in FIG. 39C, the proportion of the Ga atoms in all the atoms is2.45 atomic % to 25.22 atomic %. In the EDX mapping image in FIG. 39G,the proportion of the Ga atoms in all the atoms is 1.29 atomic % to27.64 atomi %.

FIG. 39D shows a cross-sectional EDX mapping image of Zn atoms, and FIG.39H shows a plan-view EDX mapping image of Zn atoms. In the EDX mappingimage in FIG. 39D, the proportion of the Zn atoms in all the atoms is5.05 atomic % to 23.47 atomic %. In the EDX mapping image in FIG. 39H,the proportion of the Zn atoms in all the atoms is 3.69 atomic % to27.86 atomic %.

Note that FIGS. 39A to 39D show the same region in the cross section ofSample 3A. FIGS. 39E to 39H show the same region in the plane of Sample3A.

FIGS. 40A to 40C show enlarged cross-sectional EDX mapping images ofSample 3A. FIG. 40A is an enlarged view of a part in FIG. 39B. FIG. 40Bis an enlarged view of a part in FIG. 39C. FIG. 40C is an enlarged viewof a part in FIG. 39D.

The EDX mapping images in FIGS. 40A to 40C show relative distribution ofbright and dark areas, indicating that the atoms have distributions inSample 3A. Areas surrounded by solid lines and areas surrounded bydashed lines in FIGS. 40A to 40C are examined.

As shown in FIG. 40A, a relatively dark region occupies a large area inthe area surrounded by the solid line and a relatively bright regionoccupies a large area in the area surrounded by the dashed line. Asshown in FIG. 40B, a relatively bright region occupies a large area inthe area surrounded by the solid line and a relatively dark regionoccupies a large area in the area surrounded by the dashed line.

That is, it is found that the areas surrounded by the solid lines areregions including a relatively large number of In atoms and the areassurrounded by the dashed lines are regions including a relatively smallnumber of In atoms. FIG. 40C shows that an upper portion of the areasurrounded by the solid line is relatively bright and a lower portionthereof is relatively dark. Thus, it is found that the area surroundedby the solid line is a region including In_(X2)Zn_(Y2)O_(Z2), InO_(X1),or the like as a main component.

It is found that the area surrounded by the solid line is a regionincluding a relatively small number of Ga atoms and the area surroundedby the dashed line is a region including a relatively large number of Gaatoms. As shown in FIG. 40C, a relatively bright region occupies a largearea in a right portion in the area surrounded by the upper dashed lineand a dark region occupies a large area in a left portion therein. Asshown in FIG. 40C, a relatively bright region occupies a large area inan upper left portion in the area surrounded by the lower dashed lineand a dark region occupies a large area in a lower right portiontherein. Thus, it is found that the area surrounded by the dashed lineis a region including GaO_(X3), Ga_(X4)Zn_(Y4)O_(Z4), or the like as amain component.

Furthermore, as shown in FIGS. 40A to 40C, the In atoms are relativelymore uniformly distributed than the Ga atoms, and regions includingInO_(X1) as a main component are seemingly joined to each other througha region including In_(X2)Zn_(Y2)O_(Z2) as a main component. It can bethus guessed that the regions including In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)as a main component extend like a cloud.

An In—Ga—Zn oxide having a composition in which the regions includingGaO_(X3) as a main component and the regions includingIn_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component are unevenlydistributed and mixed can be referred to as CAC-IGZO.

As shown in FIGS. 40A to 40C, each of the regions including GaO_(X3) asa main component and the regions including In_(X2)Zn_(Y2)O_(Z2) orInO_(X1) as a main component has a size of greater than or equal to 0.5nm and less than or equal to 10 nm, or greater than or equal to 1 nm andless than or equal to 3 nm.

FIGS. 41A to 41H show a cross-sectional TEM image, a plan-view TEMimage, and EDX mapping images of Sample 3J. In the EDX mapping images,the proportion of an element is indicated by grayscale: the moremeasured atoms exist in a region, the brighter the region is; the lessmeasured atoms exist in a region, the darker the region is. Themagnification of the EDX mapping images in FIGS. 41B to 41D and 41F to41H is 7,200,000 times.

FIG. 41A shows a cross-sectional TEM image, and FIG. 41E shows aplan-view TEM image. FIG. 41B is a cross-sectional EDX mapping image ofIn atoms, and FIG. 41F is a plan-view EDX mapping image of In atoms. Inthe EDX mapping image in FIG. 41B, the proportion of the In atoms in allthe atoms is 9.70 atomic % to 40.47 atomic %. In the EDX mapping imagein FIG. 41F, the proportion of the In atoms in all the atoms is 9.16atomic % to 35.76 atomic %.

FIG. 41C shows a cross-sectional EDX mapping image of Ga atoms, and FIG.41G shows a plan-view EDX mapping image of Ga atoms. In the EDX mappingimage in FIG. 41C, the proportion of the Ga atoms in all the atoms is8.23 atomic % to 31.95 atomic %. In the EDX mapping image in FIG. 41G,the proportion of the Ga atoms in all the atoms is 8.21 atomic % to28.86 atomic %.

FIG. 41D shows a cross-sectional EDX mapping image of Zn atoms, and FIG.41H shows a plan-view EDX mapping image of Zn atoms. In the EDX mappingimage in FIG. 41D, the proportion of the Zn atoms in all the atoms is5.37 atomic % to 25.92 atomic %. In the EDX mapping image in FIG. 41H,the proportion of the Zn atoms in all the atoms is 7.86 atomic % to24.36 atomic %.

Note that FIGS. 41A to 41D show the same region in the cross section ofSample 3J. FIGS. 41E to 41H show the same region in the plane of Sample3J.

In FIG. 41A, a group of laterally grown crystals is clearly observed,and in FIG. 41E, crystals having a hexagonal structure with an angle of120° is observed.

The EDX mapping images of the In atoms and the Zn atoms in FIGS. 41B and41D show particularly bright spots aligned as indicated by white lines.In FIGS. 41F and 41H, these lines form an angle of approximately 120°,which is characteristic of the hexagonal structure, and in FIGS. 41B and41D, the same layered arrangement as in FIG. 41A can be observed. Asshown in FIGS. 41C and 41G, such a tendency is not observed for Gaatoms.

The resolution of EDX is generally affected by the regularity of atomicarrangement. When atoms are regularly arranged as in a single crystal,the atoms are arranged linearly in the beam incident direction, andincident electrons are therefore channeled and propagated along the atomrows. Thus, atomic columns can be separated. In contrast, when theregularity of atomic arrangement is low, the atom rows are out ofalignment and incident electrons are therefore scattered without beingchanneled. That is, the spatial resolution is low and an obtained imageis in a blurred state in some cases.

The following consideration can be obtained. Since the crystallinity ofCAAC is not as high as that of single crystal, a beam is broadened andthe resolution of EDX mapping is not as high as that of HAADF-STEM;thus, CAAC is observed in a blurred state. From FIGS. 39A to 39H, CAC isblurred because of a broadened beam; thus, the atoms are determined tobe nanoparticles with a blurry boundary.

As described above, it is confirmed that the CAC-IGZO has a structuredifferent from that of an IGZO compound in which metal elements areevenly distributed, and has characteristics different from those of theIGZO compound. That is, it can be confirmed that in the CAC-IGZO,regions including GaO_(X3) or the like as a main component and regionsincluding In_(X2)Zn_(Y2)O_(Z2) or InO_(X1) as a main component areseparated to form a mosaic pattern.

Accordingly, it can be expected that when CAC-IGZO is used for asemiconductor element, the property derived from GaO_(X3) or the likeand the property derived from In_(X2)Zn_(Y2)O_(Z2) or InO_(X1)complement each other, whereby high on-state current (I_(on)), highfield-effect mobility (μ), and low off-state current (I_(off)) can beachieved. A semiconductor element including CAC-IGZO has highreliability. Thus, CAC-IGZO is suitably used in a variety ofsemiconductor devices typified by a display.

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

Example 4

In this example, the transistor 150 including the metal oxide 108 of oneembodiment of the present invention was fabricated and subjected totests for electrical characteristics and reliability. In this example,nine transistors, i.e., Samples 4A, 4B, 4C, 4D, 4E, 4F, 4G, 4H, and 4J,were fabricated as the transistor 150 including the metal oxide 108.

<<Structure of Samples and Fabrication Method Thereof>>

Samples 4A to 4H and 4J relating to one embodiment of the presentinvention are described below. As Samples 4A to 4H and 4J, thetransistors 150 having the structure illustrated in FIGS. 6A to 6C werefabricated by the fabrication method described in Embodiment 2 withreference to FIGS. 9A to 9D, FIGS. 10A to 10C, and FIGS. 11A to 11C.

Samples 4A to 4H and 4J were fabricated at different temperatures anddifferent oxygen flow rate ratios in formation of the metal oxide 108.The temperatures and the oxygen flow rates in formation of the metaloxide of Samples 4A to 4H and 4J are shown in Table 5 below.

TABLE 5 Formation conditions of metal oxide 108 Flow rate Formation[sccm] O₂ ratio temperature O₂ Ar [%] [° C.] Sample 4A 30 270 10 R.T.Sample 4B 90 210 30 R.T. Sample 4C 150 150 50 R.T. Sample 4D 30 270 10130 Sample 4E 90 210 30 130 Sample 4F 150 150 50 130 Sample 4G 30 270 10170 Sample 4H 90 210 30 170 Sample 4J 150 150 50 170

The samples were fabricated by the fabrication method described inEmbodiment 2. The metal oxide 108 was formed using a metal oxide target(In:Ga:Zn=1:1:1.2 [atomic ratio]).

The transistor 150 had a channel length of 2 μm and a channel width of 3μm (hereinafter, also referred to as L/W=2/3 μm).

<I_(d)-V_(g) Characteristics of Transistors>

Next, I_(d)-V_(g) characteristics of the transistors (L/W=2/3 μm) inSamples 4A to 4J were measured. As conditions for measuring theI_(d)-V_(g) characteristics of each transistor, a voltage applied to theconductive film 112 serving as a first gate electrode (hereinafter thevoltage is also referred to as gate voltage (V_(g))) and a voltageapplied to the conductive film 106 serving as a second gate electrode(hereinafter the voltage is also referred to as back gate voltage(V_(bg))) were changed from −10 V to +10 V in increments of 0.25 V. Avoltage applied to the conductive film 120 a serving as a sourceelectrode (the voltage is also referred to as source voltage (V_(s)))was 0 V (comm), and a voltage applied to the conductive film 120 bserving as a drain electrode (the voltage is also referred to as drainvoltage (V_(d))) was 0.1 V and 20 V.

In FIG. 42, the results of I_(d)-V_(g) characteristics and field-effectmobilities of Samples 4A to 4H and 4J are shown. The solid line and thedashed-dotted line represent I_(d) at V_(d)=20 V and I_(d) at V_(d)=0.1V, respectively. The dashed line and the dotted line representfield-effect mobility calculated from a value measured at V_(d)=20 V andfield-effect mobility calculated from a value measured at V_(d)=0.1 V,respectively. In FIG. 42, the first vertical axis represents I_(d) [A],the second vertical axis represents field-effect mobility (μ FE)[cm²/Vs], and the horizontal axis represents V_(g) [V].

As shown in FIG. 42, the transistors 150 of Samples 4A to 4H and 4J havenormally-off characteristics. As shown in FIG. 42, it is found thatSamples 4A to 4H and 4J have different on-state currents (I_(on)) anddifferent field effect mobilities, particularly different field effectmobilities in saturation regions. In particular, the maximum saturationmobilities and the rising characteristics of the field-effect mobilitiesaround 0 V differ distinctly.

It is found from FIG. 42 that as the substrate temperature at the timeof formation is lower or the oxygen gas flow rate ratio at the time offormation is lower, the field-effect mobility at low V_(g) issignificantly higher. In particular, Sample 4A has a maximumfield-effect mobility close to 40 cm²/Vs. Having high mobility at lowV_(g) means being suitable for high-speed driving at low voltage;therefore, application to a variety of semiconductor devices typified bya display can be expected.

From FIG. 42, different behavior of field-effect mobility is found atV_(d)=20 V (shown by dashed lines) and V_(d)=0.1 V (shown by dottedlines). As V_(g) is increased, the field-effect mobility measured atV_(d)=20 V (shown by dashed lines) becomes higher. This is considered asan influence of heat generation of the transistor. Meanwhile, thefield-effect mobility measured at V_(d)=0.1 V (shown by dotted lines) ina high V_(g) range substantially coincides with an ideal saturationmobility curve calculated by the formula (5).

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

Example 5

In this example, the analysis results of elements included in a sampleis described. For the analysis, energy dispersive X-ray spectroscopy(EDX) was performed to obtain EDX mapping images of a metal oxide of oneembodiment of the present invention formed over a substrate. An energydispersive X-ray spectrometer AnalysisStation JED-2300T manufactured byJEOL Ltd. is used as an elementary analysis apparatus in the EDXmeasurement. A Si drift detector is used to detect an X-ray emitted fromthe sample.

<<Structure of Sample and Fabrication Method Thereof>>

Sample 5A was fabricated in this example. Sample 5A includes a substrateand a metal oxide over the substrate.

Next, a method for fabricating the sample will be described.

A glass substrate was used as the substrate. Over the substrate, a100-nm-thick In—Ga—Zn oxide was formed as a metal oxide with asputtering apparatus. The formation conditions were as follows: thepressure in a chamber was 0.6 Pa; an atmosphere using an Ar gas at aflow rate of 270 sccm and an O₂ gas at a flow rate of 30 sccm assputtering gases was employed; and a metal oxide target (an atomic ratioof In:Ga:Zn=4:2:4.1) was used as a target. The metal oxide targetprovided in the sputtering apparatus was supplied with an AC power of2500 W.

Through the above steps, Sample 5A of this example was fabricated.

<Measurement Results>

In the EDX measurement, an EDX spectrum of a point is obtained in such amanner that electron beam irradiation is performed on the point in adetection target region of a sample, and the energy of characteristicX-ray of the sample generated by the irradiation and its frequency aremeasured. In this example, peaks of an EDX spectrum of the point wereattributed to electron transition to the L shell in an In atom, electrontransition to the K shell in a Ga atom, and electron transition to the Kshell in a Zn atom and the K shell in an O atom, and the proportions ofthe atoms in the point are calculated. An EDX mapping image indicatingdistributions of proportions of atoms can be obtained through theprocess in an analysis target region of a sample.

FIGS. 43A to 43D show measurement results of a cross section of Sample5A. FIG. 43A shows a cross-sectional TEM image, and FIGS. 43B and 43Cshow cross-sectional EDX mapping images. In the EDX mapping images, theproportion of an element is indicated by grayscale: the more measuredatoms exist in a region, the brighter the region is; the less measuredatoms exist in a region, the darker the region is. The magnification ofthe EDX mapping images in FIGS. 43B and 43C is 7,200,000 times. Notethat FIGS. 43A to 43C show the same region in the cross section ofSample 5A.

FIG. 43B shows a cross-sectional EDX mapping image of In atoms. In theEDX mapping image in FIG. 43B, the proportion of the In atoms in all theatoms is 12.11 atomic % to 40.30 atomic %. FIG. 43C shows across-sectional EDX mapping image of Ga atoms. In the EDX mapping imagein FIG. 43C, the proportion of the Ga atoms in all the atoms is 0.00atomic % to 13.18 atomic %.

The EDX mapping images in FIGS. 43B and 43C show relative distributionof bright and dark areas, indicating that the In atoms and the Ga atomshave distributions in Sample 5A. Here, five regions surrounded by blacklines (a region 901, a region 902, a region 903, a region 904, and aregion 905) were extracted among regions whose luminance is greater thanor equal to 75% of the maximum luminance in FIG. 43B. Five regionssurrounded by dashed lines (a region 906, a region 907, a region 908, aregion 909, and a region 910) were extracted among regions whoseluminance is greater than or equal to 75% of the maximum luminance inFIG. 43C. Five regions surrounded by white lines (a region 911, a region912, a region 913, a region 914, and a region 915) were extracted amongregions whose luminance is greater than or equal to 25% and less than orequal to 75% of the maximum luminance in FIG. 43B.

In other words, the regions 901 to 905 are regions including arelatively large number of In atoms. The regions 906 to 910 are regionsincluding a relatively large number of Ga atoms. The regions 911 to 915are regions including an average number of In atoms and Ga atoms.

Regions including a relatively large number of Ga atoms, that is, thefive regions surrounded by the dashed lines (the regions 906 to 910) inFIG. 43C are relatively dark in FIG. 43B. That is, the region includinga relatively large number of Ga atoms is expected to include arelatively small number of In atoms.

The proportions of elements in the regions 901 to 915 in FIG. 43B areshown in FIG. 43D. The regions surrounded by the black lines (theregions 901 to 905) are found to include a relatively large number of Inatoms and a relatively small number of Ga atoms. The regions surroundedby the dashed lines (the regions 906 to 910) are found to include arelatively small number of In atoms and a relatively large number of Gaatoms.

FIGS. 44A to 44D show measurement results of a plane of Sample 5A. FIG.44A shows a plan-view TEM image, and FIGS. 44B and 44C show plan-viewEDX mapping images. Note that FIGS. 44A to 44C show the same region inthe plane of Sample 5A.

FIG. 44B shows a plan-view EDX mapping image of In atoms. In the EDXmapping image in FIG. 44B, the proportion of the In atoms in all theatoms is 12.11 atomic % to 43.80 atomic %. FIG. 44C shows a plan-viewEDX mapping image of Ga atoms. In the EDX mapping image in FIG. 44C, theproportion of the Ga atoms in all the atoms is 0.00 atomic % to 14.83atomic %.

The EDX mapping images in FIGS. 44B and 44C show relative distributionof bright and dark areas, indicating that the In atoms and the Ga atomshave distributions in Sample 5A. Here, five regions surrounded by blacklines (a region 921, a region 922, a region 923, a region 924, and aregion 925) were extracted among regions whose luminance is greater thanor equal to 75% of the maximum luminance in FIG. 44B. Five regionssurrounded by dashed lines (a region 926, a region 927, a region 928, aregion 929, and a region 930) were extracted among regions whoseluminance is greater than or equal to 75% of the maximum luminance inFIG. 44C. Five regions surrounded by white lines (a region 931, a region932, a region 933, a region 934, and a region 935) were extracted amongregions whose luminance is greater than or equal to 25% and less than orequal to 75% of the maximum luminance in FIG. 44B.

Regions including a relatively large number of Ga atoms, that is, thefive regions surrounded by the dashed lines (the regions 926 to 930) inFIG. 44C are relatively dark in FIG. 44B. That is, the region includinga relatively large number of Ga atoms is expected to include arelatively small number of In atoms.

The percentages of elements in the regions 921 to 935 in FIG. 44B areshown in FIG. 44D. The regions surrounded by the black lines (theregions 921 to 925) are found to include a relatively large number of Inatoms and a relatively small number of Ga atoms. The regions surroundedby the dashed lines (the regions 926 to 930) are found to include arelatively small number of In atoms and a relatively large number of Gaatoms.

As shown in FIGS. 43D and 44D, it is found that the In atoms aredistributed in a range of higher than or equal to 25 atomic % and lowerthan or equal to 60 atomic %. Furthermore, it is found that the Ga atomsare distributed in a range of higher than or equal to 3 atomic % andlower than or equal to 40 atomic %.

A region including a relatively large number of In atoms can be expectedto have relatively high conductivity. In contrast, a region including arelatively large number of Ga atoms can be expected to have a relativelyhigh insulating property. Accordingly, it is considered that carriersflow through the region including a relatively large number of In atoms,so that the conductivity property is exhibited and high field-effectmobility (μ) is achieved. Meanwhile, it is considered that distributingthe region including a relatively large number of Ga atoms in the metaloxide enables low leakage current and favorable switching operation.

In other words, when a metal oxide having a CAC composition is used in asemiconductor element, the insulating property derived from a Ga atom orthe like and the conductivity type derived from an In atom complementeach other, whereby high on-state current (I_(on)) and high field-effectmobility (μ) can be achieved.

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

Example 6

In this example, the transistor 150 including the metal oxide 108 of oneembodiment of the present invention was fabricated and the density ofdefect states was measured. In this example, nine transistors, i.e.,Samples 6A, 6B, 6C, 6D, 6E, 6F, 6G, 6H, and 6J, were fabricated as thetransistor 150 including the metal oxide 108.

<Structure of Samples and Fabrication Method Thereof>

Samples 6A to 6H and 6J relating to one embodiment of the presentinvention are described below. As Samples 6A to 6H and 6J, thetransistors 150 having the structure illustrated in FIGS. 6A to 6C werefabricated by the fabrication method described in Embodiment 2 withreference to FIGS. 9A to 9D, FIGS. 10A to 10C, and FIGS. 11A to 11C.

Samples 6A to 6H and 6J were fabricated at different temperatures anddifferent oxygen flow rate ratios in formation of the metal oxide. Themetal oxide 108 was formed using a metal oxide target (In:Ga:Zn=1:1:1.2[atomic ratio]). The temperatures and the oxygen flow rate ratios information of the metal oxide of Samples 6A to 6H and 6J are shown inTable 6 below.

TABLE 6 Flow rate Formation [sccm] O₂ ratio temperature O₂ Ar [%] [° C.]Sample 6A 30 270 10 R.T. Sample 6B 90 210 30 R.T. Sample 6C 150 150 50R.T. Sample 6D 30 270 10 130 Sample 6E 90 210 30 130 Sample 6F 150 15050 130 Sample 6G 30 270 10 170 Sample 6H 90 210 30 170 Sample 6J 150 15050 170

The samples were fabricated by the fabrication method described inEmbodiment 2.

The transistor 150 had a channel length of 2 μm and a channel width of 3μm (hereinafter, also referred to as L/W=2/3 μm) or a channel length of2 μm and a channel width of 50 μm (hereinafter, also referred to asL/W=2/50 μm).

<Measurement of Shallow Defect States using Transistor Characteristics>

[Measurement Method of Density of Shallow Defect States]

Shallow defect states (hereinafter, also referred to as sDOS) of a metaloxide can be estimated from electrical characteristics of a transistorin which the metal oxide was used as a semiconductor. In the followingdescription, the density of interface states of the transistor wasmeasured. In addition, a method for estimating subthreshold leakagecurrent in consideration of the density of interface states and thenumber of electrons trapped by the interface states, N_(trap) isdescribed.

The number of electrons trapped by the interface states, N_(trap), canbe measured by comparing drain current-gate voltage (I_(d)-V_(g))characteristics of the transistor that was actually measured and draincurrent-gate voltage (I_(d)-V_(g)) characteristics that was calculated.

FIG. 45 illustrates ideal I_(d)-V_(g) characteristics obtained bycalculation and the actually measured I_(d)-V_(g) characteristics of thetransistor when a source voltage V_(s) is 0 V and a drain voltage V_(d)is 0.1 V. Note that only values more than or equal to 1×10⁻¹³ A at whichdrain current I_(d) can be easily measured were plotted among themeasurement results of the transistor.

A change of the drain current I_(d) with respect to the gate voltageV_(g) is more gradual in the actually measured I_(d)-V_(g)characteristics than in the ideal I_(d)-V_(g) characteristics obtainedby calculation. This is probably because an electron is trapped by ashallow interface state positioned near energy at the conduction bandminimum (represented as Ec). In this measurement, the density ofinterface states N_(it) can be estimated more accurately inconsideration of the number of electrons (per unit area and unit energy)trapped by shallow interface states, N_(trap), with use of the Fermidistribution function.

First, a method for evaluating the number of electrons trapped byinterface trap states, N_(trap), by using schematic I_(d)-V_(g)characteristics illustrated in FIG. 46 is described. The dashed lineindicates ideal I_(d)-V_(g) characteristics without trap state which areobtained by the calculation. On the dashed line, a change in gatevoltage V_(g) when the drain current changes from I_(d)1 to I_(d)2 isrepresented by ΔV_(d). The solid line indicates the actually measuredI_(d)-V_(g) characteristics. On the solid line, a change in gate voltageV_(g) when the drain current changes from I_(d)1 to I_(d)2 isrepresented by ΔV_(ex). The potential at the target interface when thedrain current is I_(d)1, the potential at the target interface when thedrain current is I_(d)2, and the amount of change are represented byϕ_(it1), ϕ_(it2), and Δϕ_(it), respectively.

The slope of the actually measured values is smaller than that of thecalculated values in FIG. 46, which indicates that ΔV_(ex) is alwayslarger than ΔV_(id). Here, a difference between ΔV_(ex) and ΔV_(id)corresponds to a potential difference that is needed for trapping of anelectron in a shallow interface state. Therefore, ΔQ_(trap) which is theamount of change in charge due to trapped electrons can be expressed bythe following formula (6).

[Formula 6]ΔQ _(trap) =−C _(tg)(ΔV _(ex) −ΔV _(id))  (6)

Ct_(g) is combined capacitance of an insulator and a semiconductor perunit area. In addition, ΔQ_(trap) can be expressed by the formula (7) byusing the number of trapped electrons N_(trap) (per unit area and perunit energy). Note that q represents elementary charge.

[Formula 7]ΔQ _(trap) =−qN _(trap)Δϕ_(it)  (7)

Simultaneously solving the formulae (6) and (7) gives the formula (8).

[Formula 8]−C _(tg)(ΔV _(ex) −ΔV _(id))=qN _(trap)Δϕ_(it)  (8)

Then, taking the limit zero of Δϕ_(it) in the formula (8) gives theformula (9).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 9} \right\rbrack\mspace{625mu}} & \; \\{N_{trap} = {{\frac{C_{tg}}{q}{\lim\limits_{{\Delta\phi}_{it}\rightarrow 0}\left( {\frac{\Delta\; V_{ex}}{{\Delta\phi}_{it}} - \frac{\Delta\; V_{id}}{{\Delta\phi}_{it}}} \right)}} = {C_{tg}\left( {\frac{\partial V_{ex}}{\partial\phi_{it}} - \frac{\partial V_{id}}{\partial\phi_{it}}} \right)}}} & (9)\end{matrix}$

In other words, the number of electrons trapped by an interface,N_(trap), can be estimated by using the ideal I_(d)-V_(g)characteristics, the actually measured I_(d)-V_(g) characteristics, andthe formula (9). Furthermore, the relationship between the drain currentand the potential at the interface can be obtained by the abovecalculations.

The relationship between the number of electrons N_(trap) per unit areaand per unit energy and the density of interface states N_(it) isexpressed by the formula (10).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 10} \right\rbrack\mspace{599mu}} & \; \\{N_{trap} = {\frac{\partial}{\partial\phi_{it}}{\int_{- \infty}^{\infty}{{N_{it}(E)}{f(E)}{dE}}}}} & (10)\end{matrix}$

Here, f(E) is Fermi distribution function. The N_(trap) obtained fromthe formula (9) is fitted with the formula (10) to determine N_(it). Theconduction characteristics including I_(d)<0.1 pA can be obtained by thedevice simulator to which the N_(it) is set.

The actually measured I_(d)-V_(g) characteristics in FIG. 45 are appliedto the formula (9) and the results of extracting N_(trap) are plotted aswhite circles in FIG. 47. The vertical axis in FIG. 47 represents Fermienergy Ef at the conduction band minimum Ec of a semiconductor. Themaximum value is positioned on the dashed line just under Ec. When taildistribution of the formula (11) is assumed as N_(it) of the formula(10), N_(trap) can be fitted well like the dashed line in FIG. 47. As aresult, the trap density at an end of the conduction bandN_(ta)=1.67×10¹³ cm⁻² eV and the characteristic decay energyW_(ta)=0.105 eV are obtained as the fitting parameters.

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 11} \right\rbrack\mspace{599mu}} & \; \\{{N_{it}(E)} = {N_{ta}{\exp\left\lbrack \frac{E - E_{c}}{W_{ta}} \right\rbrack}}} & (11)\end{matrix}$

FIGS. 48A and 48B show the inverse calculation results of I_(d)-V_(g)characteristics by feeding back the obtained fitting curve of interfacestate to the calculation using the device simulator. FIG. 48A shows thecalculated I_(d)-V_(g) characteristics when the drain voltage V_(d) is0.1 V and 1.8V and the actually measured I_(d)-V_(g) characteristicswhen the drain voltage V_(d) is 0.1 V and 1.8V. FIG. 48B is a graph inwhich the drain current I_(d) is a logarithm in FIG. 48A.

The curve obtained by the calculation substantially matches with theplot of the actually measured values, which suggests that the calculatedvalues and the measured values are highly reproducible. Thus, the abovemethod is quite appropriate as a method for calculating the density ofshallow defect states.

[Measurement Results of Density of Shallow Defect States]

Next, the density of shallow defect states of Samples 6A, 6B, 6C, 6D,6E, 6F, 6G, 6H, and 6J were measured by comparing measured electricalcharacteristics with ideal calculation values according to theabove-described method.

FIG. 49 shows calculated average density of shallow defect states ofSamples 6A to 6H and 6J.

As shown in FIG. 49, the sample formed at a lower oxygen flow rate ratioin formation of the metal oxide 108 or a lower temperature in formationof the metal oxide 108 has a lower peak density of shallow defectstates.

As described above, Samples 6A to 6H and 6J are found to be transistorsincluding a metal oxide film with a low density of defect states. It isinferred that the oxygen-transmitting property is improved because themetal oxide film is formed at a low temperature and a low oxygen flowrate ratio, and the amount of diffused oxygen in the fabrication processof the transistor is increased, whereby the amount of defects such asoxygen vacancies in the metal oxide film and at the interface betweenthe metal oxide film and the insulating film is reduced.

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

Example 7

In this example, the transistor 150 including the metal oxide 108 of oneembodiment of the present invention was fabricated and the density ofdefect states was measured. In this example, nine transistors, i.e.,Samples 7A, 7B, 7C, 7D, 7E, 7F, 7G, 7H, and 7J, were fabricated as thetransistor 150 including the metal oxide 108.

<Structure of Samples and Fabrication Method Thereof>

Samples 7A to 7H and 7J relating to one embodiment of the presentinvention are described below. As Samples 7A to 7H and 7J, thetransistors 150 having the structure illustrated in FIGS. 6A to 6C werefabricated by the fabrication method described in Embodiment 2 withreference to FIGS. 9A to 9D, FIGS. 10A to 10C, and FIGS. 11A to 11C.

Samples 7A to 7H and 7J were fabricated at different temperatures anddifferent oxygen flow rate ratios in formation of the metal oxide 108.The metal oxide 108 was formed using a metal oxide target(In:Ga:Zn=4:2:4.1 [atomic ratio]). The temperatures and the oxygen flowrate ratios in formation of the metal oxide of the samples 7A to 7H and7J are shown in Table 7 below.

TABLE 7 Flow rate Formation [sccm] O₂ ratio temperature O₂ Ar [%] [° C.]Sample 7A 30 270 10 R.T. Sample 7B 90 210 30 R.T. Sample 7C 300 0 100R.T. Sample 7D 30 270 10 130 Sample 7E 90 210 30 130 Sample 7F 300 0 100130 Sample 7G 30 270 10 170 Sample 7H 90 210 30 170 Sample 7J 300 0 100170

The samples were fabricated by the fabrication method described inEmbodiment 2.

The transistor 150 had a channel length of 2 μm and a channel width of 3μm (hereinafter, also referred to as L/W=2/3 μm) or a channel length of2 μm and a channel width of 50 μm (hereinafter, also referred to asL/W=2/50 μm).

<Measurement of Shallow Defect States using Transistor Characteristics>

[Measurement Method of Density of Shallow Defect States]

Shallow defect states of the metal oxide 108 were estimated fromelectrical characteristics of a transistor in which the metal oxide wasused as a semiconductor. The calculation method was similar to thatdescribed in the above example. The density of interface states of thetransistor was measured. In addition, subthreshold leakage current wasestimated in consideration of the density of interface states and thenumber of electrons trapped by the interface states, N_(trap).

[Measurement Results of Density of Shallow Defect States]

Next, the density of shallow defect states of Samples 7A, 7B, 7C, 7D,7E, 7F, 7G, 7H, and 7J were measured by comparing measured electricalcharacteristics with ideal calculation values according to theabove-described method.

FIG. 50 shows calculated average density of shallow defect states ofSamples 7A to 7H and 7J.

As shown in FIG. 50, the sample formed at a lower oxygen flow rate ratioin formation of the metal oxide 108 or a lower temperature in formationof the metal oxide 108 has a lower peak density of shallow defectstates.

As described above, Samples 7A to 7H and 7J are found to be transistorsincluding a metal oxide film with a low density of defect states. It isinferred that the oxygen-transmitting property is improved because themetal oxide film is formed at a low temperature and a low oxygen flowrate ratio, and that the amount of diffused oxygen in the fabricationprocess of the transistor is increased, whereby the amount of defectssuch as oxygen vacancies in the metal oxide film and at the interfacebetween the metal oxide film and the insulating film is reduced.

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

Example 8

In this example, the transistor 150 including the metal oxide 108 of oneembodiment of the present invention was fabricated and subjected totests for electrical characteristics and reliability. In this example, atransistor of Sample 8A was fabricated as the transistor 150 includingthe metal oxide 108.

<Structure of Sample and Fabrication Method Thereof>

Sample 8A relating to one embodiment of the present invention isdescribed below. As Sample 8A, the transistor 150 having the structureillustrated in FIGS. 6A to 6C was fabricated by the fabrication methoddescribed in Embodiment 2 with reference to FIGS. 9A to 9D, FIGS. 10A to10C, and FIGS. 11A to 11C.

The temperature and the oxygen flow rate ratio in formation of the metaloxide 108 of Sample 8A are shown in Table 8 below.

TABLE 8 Flow rate Formation [sccm] O₂ ratio temperature O₂ Ar [%] [° C.]Sample 8A 0 300 0 R.T.

Sample 8A was fabricated by the fabrication method described inEmbodiment 2. The metal oxide 108 was formed using a metal oxide target(In:Ga:Zn=5:1:7 [atomic ratio]).

The transistor 150 had a channel length of 3 μm and a channel width of50 μm (hereinafter, also referred to as L/W=3/50 μm).

<I_(d)-V_(g) Characteristics of Transistor>

Next, I_(d)-V_(g) characteristics of the transistor (L/W=3/50 μm) inSample 8A were measured. As conditions for measuring the I_(d)-V_(g)characteristics of the transistor, a voltage applied to the conductivefilm 112 serving as a first gate electrode (hereinafter the voltage isalso referred to as gate voltage (V_(g))) and a voltage applied to theconductive film 106 serving as a second gate electrode (hereinafter thevoltage is also referred to as back gate voltage (V_(bg))) were changedfrom −10 V to +10 V in increments of 0.25 V. A voltage applied to theconductive film 120 a serving as a source electrode (the voltage is alsoreferred to as source voltage (V_(s))) was 0 V (comm), and a voltageapplied to the conductive film 120 b serving as a drain electrode (thevoltage is also referred to as drain voltage (V_(d))) was 0.1 V and 20V.

In FIG. 51, the results of I_(d)-V_(g) characteristics and field-effectmobility of Sample 8A are shown. The solid line and the dashed-dottedline represent I_(d) at V_(d)=20 V and I_(d) at V_(d)=0.1 V,respectively. The dashed line represents field-effect mobility. In FIG.51, the first vertical axis represents I_(d) [A], the second verticalaxis represents field-effect mobility (μ FE) [cm²/Vs], and thehorizontal axis represents V_(g) [V]. The field-effect mobility wascalculated from the value measured at V_(d)=20 V.

Note that the results in FIG. 51 were obtained with the upper limit ofI_(d) in the measurement set to 1 mA. In FIG. 51, when V_(d) is 20 V,I_(d) exceeds this upper limit at V_(g)=7.5 V. For this reason, FIG. 51shows the field-effect mobility in the range where V_(g) is lower thanor equal to 7.5 V as the field-effect mobility estimated from suchI_(d)-V_(g) characteristics.

As shown in FIG. 51, the transistor of this example has favorableelectrical characteristics. Here, Table 9 shows the transistorcharacteristics that are shown in FIG. 51.

TABLE 9 μFE (max) Ioff μFE (@Vg = 2 V) μFE (max)/ [cm²V⁻¹s⁻¹] Vth [V] S[V/decade] [A/cm²] [cm²V⁻¹s⁻¹] μFE (@Vg = 2 V) 103 −0.1 0.12 <1 × 10⁻¹²70 1.47

As described above, the field-effect mobility of the transistor of thisexample exceeds 100 cm²/Vs. This field-effect mobility is equivalent tothat of a transistor including low-temperature polysilicon and meansextraordinary characteristics for a transistor using the metal oxide108.

As shown in Table 9, Sample 8A includes a first region where the maximumvalue of the field-effect mobility of the transistor at a gate voltageof higher than 0 V and lower than or equal to 10 V is larger than orequal to 60 cm²/Vs and smaller than 150 cm²/Vs, a second region wherethe threshold voltage is higher than or equal to −1 V and lower than orequal to 1 V, a third region where the S value is smaller than 0.3V/decade, and a fourth region where the off-state current is lower than1×10⁻¹² A/cm², and μ_(FE)(max)/μ_(FE)(V_(g)=2 V) is larger than or equalto 1 and smaller than 2 where μ_(FE)(max) represents the maximum valueof the field-effect mobility of the transistor and μ_(FE)(V_(g)=2 V)represents the value of the field-effect mobility of the transistor at agate voltage of 2 V.

The characteristics of the transistor can be obtained with the use ofthe metal oxide 108 described above. A transistor including the metaloxide 108 in its semiconductor layer can have both high carrier mobilityand excellent switching characteristics.

At least part of this example can be implemented in combination with anyof the other embodiments and the other examples described in thisspecification as appropriate.

REFERENCE NUMERALS

001: region, 002: region, 100: transistor, 102: substrate, 104:insulating film, 106: conductive film, 108: metal oxide, 108 a: metaloxide, 108 n: region, 110: insulating film, 110_0: insulating film, 112:conductive film, 112_0: conductive film, 112_1: conductive film, 112_2:conductive film, 116: insulating film, 118: insulating film, 120 a:conductive film, 120 b: conductive film, 122: insulating film, 140:mask, 141 a: opening, 141 b: opening, 143: opening, 150: transistor,160: transistor, 300A: transistor, 300B: transistor, 300C: transistor,300D: transistor, 302: substrate, 304: conductive film, 306: insulatingfilm, 307: insulating film, 308: metal oxide, 312 a: conductive film,312 b: conductive film, 314: insulating film, 316: insulating film, 318:insulating film, 320 a: conductive film, 320 b: conductive film, 341 a:opening, 341 b: opening, 342 a: opening, 342 b: opening, 342 c: opening,600: display panel, 601: transistor, 604: connection portion, 605:transistor, 606: transistor, 607: connection portion, 612: liquidcrystal layer, 613: conductive film, 617: insulating film, 620:insulating film, 621: insulating film, 623: conductive film, 631:coloring layer, 632: light-blocking film, 633 a: alignment film, 633 b:alignment film, 634: coloring layer, 635: conductive film, 640: liquidcrystal element, 641: adhesive layer, 642: adhesive layer, 643:conductive film, 644: EL layer, 645 a: conductive film, 645 b:conductive film, 646: insulating film, 647: insulating film, 648:conductive film, 649: connection layer, 651: substrate, 652: conductivefilm, 653: semiconductor film, 654: conductive film, 655: opening, 656:polarizing plate, 659: circuit, 660: light-emitting element, 661:substrate, 662: display portion, 663: conductive film, 666: wiring, 672:FPC, 673: IC, 681: insulating film, 682: insulating film, 683:insulating film, 684: insulating film, 685: insulating film, 686:connector, 687: connection portion, 700: model, 702: local structure,704: local structure, 706: local structure, 708: local structure, 710:local structure, 712: local structure, 901: region, 902: region, 903:region, 904: region, 905: region, 906: region, 907: region, 908: region,909: region, 910: region, 911: region, 912: region, 913: region, 914:region, 915: region, 920: region, 921: region, 922: region, 923: region,924: region, 925: region, 926: region, 927: region, 928: region, 929:region, 930: region, 931: region, 932: region, 933: region, 934: region,and 935: region.

This application is based on Japanese Patent Application Serial No.2016-111984 filed with Japan Patent Office on Jun. 3, 2016, JapanesePatent Application Serial No. 2016-125756 filed with Japan Patent Officeon Jun. 24, 2016, Japanese Patent Application Serial No. 2016-126116filed with Japan Patent Office on Jun. 25, 2016, Japanese PatentApplication Serial No. 2016-135510 filed with Japan Patent Office onJul. 7, 2016, Japanese Patent Application Serial No. 2016-151723 filedwith Japan Patent Office on Aug. 2, 2016, Japanese Patent ApplicationSerial No. 2016-155888 filed with Japan Patent Office on Aug. 8, 2016,Japanese Patent Application Serial No. 2016-184961 filed with JapanPatent Office on Sep. 22, 2016, and Japanese Patent Application SerialNo. 2016-189426 filed with Japan Patent Office on Sep. 28, 2016, theentire contents of which are hereby incorporated by reference.

The invention claimed is:
 1. A field-effect transistor comprising ametal oxide, wherein a channel formation region of the transistorcomprises a material having at least two different energy band widths,wherein the material comprises nano-size particles each with a size ofgreater than or equal to 0.5 nm and less than or equal to 10 nm, whereina first nano-size particle of the nano-size particles comprises In,wherein a second nano-size particle of the nano-size particles comprisesGa, and wherein the nano-size particles are dispersed or distributed ina mosaic pattern.
 2. A field-effect transistor comprising a metal oxide,wherein a channel formation region of the transistor comprises amaterial having at least two different energy band widths, wherein thematerial comprises nano-size particles each with a size of greater thanor equal to 0.5 nm and less than or equal to 10 nm, wherein a firstnano-size particle of the nano-size particles comprises In, wherein asecond nano-size particle of the nano-size particles comprises Ga,wherein the nano-size particles are dispersed or distributed in a mosaicpattern, and wherein the channel formation region comprises a paththrough which carriers easily flow from a source of the transistor to adrain of the transistor.
 3. A field-effect transistor comprising a metaloxide, wherein a channel formation region of the transistor comprises amaterial having at least two different energy band widths, wherein thematerial comprises nano-size particles each with a size of greater thanor equal to 0.5 nm and less than or equal to 10 nm, wherein a firstnano-size particle of the nano-size particles comprises In, wherein asecond nano-size particle of the nano-size particles comprises Ga, andwherein the nano-size particles are dispersed or distributed in a mosaicpattern, wherein in the case where a gate voltage is applied to thetransistor in a forward direction, the material having at least twodifferent energy band widths each has a first function of allowingcarriers to flow from a source of the transistor to a drain of thetransistor, and wherein in the case where a gate voltage is applied tothe transistor in a reverse direction, the material having at least twodifferent energy band widths each has a second function of suppressingcarriers from flowing from the source to the drain.
 4. The field-effecttransistor according to claim 3, wherein the nano-size particlescomprise a region where their surroundings are blurred and overlap witheach other.
 5. The field-effect transistor according to claim 3, whereinthe first function makes the material having at least two differentenergy band widths each narrower than an energy band width in the casewhere the gate voltage is held at 0 V, and wherein the second functionmakes the material having at least two different energy band widths eachwider than an energy band width in the case where the gate voltage isheld at 0 V.
 6. The field-effect transistor according to claim 5,wherein the nano-size particles comprise a region where theirsurroundings are blurred and overlap with each other.
 7. A field-effecttransistor comprising: a channel formation region comprising a metaloxide, the metal oxide comprising a first region and a second region,wherein the first region comprises indium, gallium, and zinc, whereinthe second region comprises indium, gallium, and zinc, wherein thenumber of gallium atom in the first region is larger than the number ofgallium atom in the second region, and wherein each of a size of thefirst region and a size of the second region in energy dispersive X-rayspectroscopy (EDX) mapping image is greater than or equal to 0.5 nm andless than or equal to 3 nm.
 8. A field-effect transistor comprising: achannel formation region comprising a metal oxide, the metal oxidecomprising a first region and a second region, wherein the first regioncomprises indium, gallium, and zinc, wherein the second region comprisesindium, gallium, and zinc, wherein the number of gallium atom in thefirst region is larger than the number of gallium atom in the secondregion, wherein the number of indium atom in the first region is smallerthan the number of indium atom in the second region, and wherein each ofa size of the first region and a size of the second region in energydispersive X-ray spectroscopy (EDX) mapping image is greater than orequal to 0.5 nm and less than or equal to 3 nm.